fix(web): flush pending raw-markdown on unload (BUG-2024)

[xarmian]

BUG-2024 — Raw-markdown editor loses typed content on tab close/reload

From the PLAN-1984 Web-UX audit: up to ~1.2s of typed raw-markdown content is lost on tab close/reload. The item-detail beforeunload handler only flushed the collaborative (Yjs) path and ignored pending raw markdown debounced edits, and never warned on dirty state.

Before

• Raw-markdown typing is debounced (1200ms) before it PATCHes items.content.
• On tab close/reload within that window, onBeforeUnload flushed only the collab Y.Doc snapshot — the pending raw edit (rawPendingMarkdown) was silently dropped.
• No native "unsaved changes" prompt.

After

• onBeforeUnload still flushes the collab path unchanged, and now additionally, when raw markdown is dirty (rawPendingMarkdown !== null):
  • Fires an immediate keepalive PATCH /items/{id} of the pending content so the request survives page teardown (keepalive: true, the modern sendBeacon equivalent; markdown bodies are well under the ~64KB cap).
  • Cancels the queued debounce timer so it can't fire a second, older-content PATCH racing the keepalive one.
  • Sets event.preventDefault() + event.returnValue so the browser shows its native "unsaved changes" prompt — only when actually dirty, never on a clean page.
  • Clears rawPendingMarkdown + the dirty flag once the keepalive save lands, so if the user picks "Stay" a later unload doesn't re-prompt / re-PATCH already-saved content.

rawPendingMarkdown is the authoritative dirty signal — set on every raw keystroke, cleared to null after each normal debounced save. So the prompt only fires when there's genuinely unsaved content.

client.ts

api.items.update gains an optional { keepalive?: boolean } — passed straight to fetch (mirrors the existing flushCollabContent keepalive option). No behavior change for existing callers.

Test plan

• npm run check — 0 errors (only pre-existing unrelated warnings)
• npm run build — succeeds
• npm run test — 138/138 pass
• Handler reasoned through: keepalive PATCH + returnValue fire only when rawPendingMarkdown !== null; dirty cleared after normal save (existing path) and after keepalive save; collab flush preserved untouched.

Refs BUG-2024, PLAN-1984.