Doc trees phase 2: create/rename/delete empty user trees

[ddisisto]

Add the UX and CRUD for user-created tree containers. Depends on #9 (the trees store and tree-aware doc-list.ts must exist first). No doc import yet — user trees in this phase are empty labeled folders. The focus is proving out the interaction model (inline-input rows, two-click-confirm) before content primitives land in #11.

Interaction model

From the #8 planning discussion (D5): inline-input rows, matching the lightweight pattern already used by the stats-reset cell at doc-list.ts:273-291. No modals, no popovers.

Creating a tree

• A new <+> new tree row renders at the root level of the doc-list panel, below the existing about tree.
• Click → the row becomes an inline <input> with a placeholder ("tree name").
• Enter commits: generates id: 'u:<uuid>', calls putTree({ id, label, kind: 'user', created }), refreshes the panel.
• Escape, blur, or outside-click cancels and restores the <+> new tree row.
• Empty-string commits are treated as cancel.
• After commit, the new tree renders as a labeled folder with no children (no <+> add row yet — that lands in Doc trees phase 2: create/rename/delete empty user trees #11).

Renaming a tree

• Two-click-confirm on the user tree's label row (matching the reset-cell pattern):
  • First click arms a "rename?" affordance on the row with a 3s timeout.
  • Second click (within the timeout) swaps the label for an inline input pre-filled with the current name.
  • Enter commits, Escape/blur cancels.
• System trees (kind: 'system') do not expose rename — no click handler, no armed state. about stays about.

Deleting a tree

• Separate two-click-confirm on a small control in the tree row (e.g. a × that appears on hover, or a dedicated cell — match whatever feels consistent with the existing stats-reset affordance).
• Confirmed delete cascades: deleteTree(id) plus deleteDocumentData() for each doc in the tree. (Trees in this phase are empty, so the cascade is a no-op, but the code path needs to be in place for Doc trees phase 2: create/rename/delete empty user trees #11.)
• about does not expose delete.

Data model

Already set up in #9 — this issue just uses:

• putTree, listTrees, deleteTree from store.ts
• StoredDocument.treeId for the cascade scope

No new schema.

Acceptance

• <+> new tree row renders at the root of the doc-list panel.
• Clicking it produces an inline input; Enter commits, Escape/blur/outside-click cancels.
• Committed trees persist across reloads.
• User trees render as labeled folders, distinct from about visually only insofar as they have no children yet.
• Two-click rename works on user trees; inline input replaces the label; Enter commits.
• Two-click delete works on user trees; cascades via deleteTree + deleteDocumentData.
• about exposes neither rename nor delete — no affordances, no armed states.
• Multiple user trees can coexist and are ordered deterministically (creation order is fine; document in the issue whatever you pick).

Touchpoints

• frontend/src/doc-list.ts — new inline-input row component, <+> new tree row, rename/delete affordances on user-tree rows
• frontend/src/styles.css — styling for the new rows and the armed states
• frontend/src/store.ts — no changes expected (CRUD landed in Docs overhaul #9)

Out of scope

• <+> add child rows inside a user tree (lands in Doc trees phase 2: create/rename/delete empty user trees #11)
• Any form of doc import
• Reordering trees (deterministic creation-order is fine for now)

Parent: #8