fix(schedule): derive the item editor's selection from the document

[atman-33]

Problem

In the Schedule tab, an edit made in the side panel could undo a change made on
the grid.

Reported case: create a bar for 7/5–7/6, shorten it to 7/5, then change its
kind to arrow — the period jumps back to 7/5–7/6.

Root cause

The selected element was held as a copy in two places while doc was the real
source of truth:

• schedule-view.tsx — const [selected, setSelected] = useState<ScheduleItem | null>(null)
• item-editor.tsx — const [draft, setDraft] = useState(item), re-seeded from
  the prop by a useEffect

Every edit path that does not run through the side panel updates doc alone and
leaves both copies stale: a drag or resize on the grid (ScheduleGrid suppresses
onSelectItem when the pointer moved, so the selection is never refreshed), an
undo, or a reload after an Obsidian / AI-agent edit.

The editor's commit then hands { ...draft, ...patch } — the whole stale
element — to patchItem, writing the pre-drag values back over the document.

The reported case is one instance. The same revert happened for title, colour and
task-link edits after any grid gesture.

Fix

Make the selection a derived value instead of a copy, so the staleness cannot
exist:

• schedule-view.tsx stores only selectedId; the element is looked up in
  doc.items with useMemo. All call sites follow (file switch, undo/redo,
  Escape, Delete, grid selection, editor callbacks, element creation), and the
  arrow-key nudges now call patchItem alone.
• item-editor.tsx renders straight from its item prop — the local draft state
  and its re-seeding effect are gone. commit builds { ...item, ...patch };
  the normalisation (start/end inversion guard, end = start for point kinds) is
  unchanged.
• Recorded the pattern in .claude/rules/ui-conventions.md so future side panels
  do not reintroduce it.

Intended side effects: the panel now follows external file edits, and it closes
itself when the selected element is deleted.

Verification

• npm run build — passed
• npm test — 124/124 passed (10 files)

No component test added: the repo has no jsdom/testing-library setup, only
node-environment vitest for src/lib. Manual checks:

1. Bar 7/5–7/6 → drag its right edge to 7/5 → switch kind to arrow: the period
   stays 7/5.
2. Drag a bar sideways, then edit its title: the position does not jump back.
3. Edit the note in Obsidian with the panel open: the panel shows the new values
   after the reload.

🤖 Generated with Claude Code

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