Flush the description's pending autosave on every close path

[dmnyc]

Fixes #342. Stacked on #341 — review and merge that first; this PR's base is feat/imeta-alt-text, so the diff shown here is just this fix.

The gap

The ALT row committed on its three own exits (Save description, Escape, trash) — but pages closed it for their own reasons, and each dropped the last half-second of the debounced autosave:

• chip toggle (re-tap the chip to close the row)
• the review window starting, both composers — worst case: Post pressed mid-word and the note publishes without the tail of the description
• the tab's account switch (the old account's draft was persisted without it)

The fix

• buildAltEditorRow exposes flushPending() on the row: fires the pending onChange if the 500ms debounce is live.
• closeAltEditor(flush) flushes by default.
• Two closes deliberately don't: image removal passes false (the slot is gone — flushing would write one image's words onto another), and the drag closes before the splice so the flush still lands in the slot the row was opened for.
• The panel's Post press closes-with-flush at the top of the handler; the tab's account switch closes before flushDraft(), the old account's last chance to take the tail.

Tests pin all of it: the exposed flush, the flush !== false guard, removal's false, drag-before-splice ordering, and the two load-bearing call sites.