Feature Request: Support independent/custom note titles

[yifjfgup]

Problem Statement

Currently, a note's title is strictly derived from the first line of its body
(Note.derivedTitle(from: body)), and any change to the body text
unconditionally overwrites the title.

This makes it difficult to assign custom, concise, or distinct titles to notes
without altering the actual body text formatting.

Proposed Solution

1. Support Custom Title with Dynamic Fallback:
   • Allow setting an explicit custom title on a note.
   • If a custom title is set, body edits should preserve the custom title.
   • If the custom title is cleared (or empty), automatically fall back to the
     dynamic first-line derived title (and "New note" / note.untitled if
     empty).
2. Inline Title Editing:
   • Replace the static title text in NoteEditorView's header with an inline
     TextField that supports debounced autosave.
   • Pressing Return confirms the title and focuses the text editor
     seamlessly.
   • Provide a context menu option ("Reset to Auto Title") to quickly revert
     back to the derived title.
3. Library View Integration:
   • Allow editing note titles directly in the LibraryDetail pane.
   • Update search to match both title and body text.

[aimen08]

Shipped in v1.6.0 via your own #29 🎉 — thanks for the feature and the implementation. Merged with two follow-ups on top: a one-time migration so pre-existing notes (whose stored titles equalled their derived first lines) keep auto-renaming instead of freezing, and the title fields draw in each note's ink — the system placeholder colour rendered white on the paper in dark mode.