empty: Add composable empty states (longbridge#3030)

Closes longbridge#2862

## Description

- Add six stateless, composable parts: `Empty`, `EmptyHeader`,
`EmptyMedia`, `EmptyTitle`, `EmptyDescription`, and `EmptyContent`.
- Provide optional typed header/content and media/title/description
slots with replacement semantics and stable rendering order. Extra root
children follow the named slots.
- Keep every part independently styleable, with theme-derived
colors/radii and rem-based spacing. Media accepts icons, avatars, avatar
groups, and arbitrary elements; actions and retained input state stay in
the application. No Base-layer changes, dependencies, or
component-specific global theme fields are added.
- Add Story examples for default actions, outline/background styling,
avatars, avatar groups, search results, embedded inputs, constrained
layouts, and minimal content, plus matching English and Chinese
documentation.
- Cover builders and real component composition. The layout regression
catches zero-width avatar groups and verifies centered/leading
alignment, two rem sizes, and named-slot ordering.

- Register all six Empty parts in Component Shell, preserve typed named
slots and independently styled parts, and add the inventory entries and
a JavaScript Story with application-owned actions and retained input
state.
- Keep element arguments intact across the shared named-slot bridge so
registered `header`/`content` methods receive the same Element values as
other registered methods. Existing built-in slots retain their ownership
checks.
- Remove the optional external reference links from both Empty
documentation pages so the generated documentation bundle passes its
canonical-route check without changing that check.

Composition and spacing reference the shadcn/ui Empty Nova examples
while retaining GPUI Component's theme and native controls.

## Screenshot

Screenshots are not attached. Visual verification was completed in the
signed macOS Story app: individual avatars and avatar groups are
centered, constrained Chinese text wraps without clipping, and Empty
follows the theme's font-size and corner-radius controls.

## Breaking Changes

None. The API is additive; existing imports and consumers remain
unchanged. Applications can optionally adopt named parts in place of
their own placeholder markup (this is not a required migration):

```diff
- div().child("No projects yet")
+ Empty::new().header(
+     EmptyHeader::new().title(EmptyTitle::new().child("No projects yet")),
+ )
```

The new types are imported from `gpui_kit::component::empty`; GPUI's
existing `gpui_kit::Empty` remains unchanged.

## How to Test

Passed:

```sh
cargo build --locked -p gpui-component -p gpui-component-story
cargo test --locked -p gpui-component --lib empty::tests
cargo clippy --locked -p gpui-component -p gpui-component-story --all-targets -- --deny warnings
cargo test --locked -p gpui-component-shell --all-targets
cargo test --locked -p gpui-shell --lib tests::render
cargo clippy --locked -p gpui-component-shell --all-targets -- --deny warnings
GPUI_COMPONENT_SHELL_BIN=target/debug/gpui-component-shell node crates/component-shell/tests/types/run.mjs
node examples/js_story/fixtures/verify-coverage.mjs
rustfmt --edition 2024 --check crates/component/src/empty.rs crates/story/src/stories/empty_story.rs
typos crates/component/src/empty.rs crates/story/src/stories/empty_story.rs website/component/empty.md website/zh-CN/component/empty.md
git diff --check
```

Native verification used `./script/run-story-macos Empty`:

- Create project shows `Untitled project`; Reset example restores the
empty state.
- Clear search removes `Archive` and restores all results; typing
`Website` immediately afterward filters the list without refocusing the
field.
- The input embedded in EmptyContent edits independently of the
project-search input.
- Avatar and AvatarGroup centering, leading-aligned constrained content,
long Chinese text, larger font size, and zero/default theme radii were
checked visually.

Both Markdown files parse, and their eight Rust code blocks match. The
production `buildLlmsContent` output passes the canonical-route
assertion and includes both Empty pages. The new host tests cover wrong
slot types, replacement semantics, standalone parts, styles, and a
working child-button callback; all registered Story examples
materialize. Shell CLI tests were run with macOS desktop-service access.
The full documentation-site build and Windows/Linux native runs were not
performed.

## Checklist

- [x] Read CONTRIBUTING and followed the component/design guides.
- [x] Reviewed the scoped API and implementation; this component, Story,
documentation, and regression coverage were AI-assisted.
- [x] Ran the affected Story using the documented signed macOS launch
script.
- [x] No platform-specific implementation changes.

---------

Co-authored-by: Jason Lee <huacnlee@gmail.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people

crates/component-shell/component-inventory.json

@@ -346,6 +346,55 @@
       "classification": "infrastructure",
       "explanation": "Dock is an application layout subsystem already exposed directly by gpui-shell; it is not a singular gpui-component constructor."
     },
+    {
+      "source": "ui",
+      "name": "empty",
+      "classification": "component",
+      "registration": {
+        "status": "registered",
+        "descriptor": "Empty",
+        "exports": [
+          "Empty"
+        ],
+        "related": [
+          {
+            "descriptor": "EmptyHeader",
+            "exports": [
+              "EmptyHeader"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyMedia",
+            "exports": [
+              "EmptyMedia"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyTitle",
+            "exports": [
+              "EmptyTitle"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyDescription",
+            "exports": [
+              "EmptyDescription"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyContent",
+            "exports": [
+              "EmptyContent"
+            ],
+            "role": "named-slot-part"
+          }
+        ]
+      }
+    },
     {
       "source": "ui",
       "name": "form",
@@ -1477,6 +1526,55 @@
         ]
       }
     },
+    {
+      "source": "story",
+      "name": "empty",
+      "classification": "component",
+      "registration": {
+        "status": "registered",
+        "descriptor": "Empty",
+        "exports": [
+          "Empty"
+        ],
+        "related": [
+          {
+            "descriptor": "EmptyHeader",
+            "exports": [
+              "EmptyHeader"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyMedia",
+            "exports": [
+              "EmptyMedia"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyTitle",
+            "exports": [
+              "EmptyTitle"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyDescription",
+            "exports": [
+              "EmptyDescription"
+            ],
+            "role": "named-slot-part"
+          },
+          {
+            "descriptor": "EmptyContent",
+            "exports": [
+              "EmptyContent"
+            ],
+            "role": "named-slot-part"
+          }
+        ]
+      }
+    },
     {
       "source": "story",
       "name": "form",