refactor(web): extract contentSaver + shared progress-merge from monoliths (TASK-2029)

[xarmian]

TASK-2029 (PLAN-1984 Web-UX audit)

Behavior-preserving refactor. Pulls two tangled clusters out of the item/collection page monoliths into cohesive, unit-tested modules so future feature work can touch them safely.

A. contentSaver.svelte.ts — raw-markdown saver (item detail page)

New web/src/lib/items/contentSaver.svelte.ts — createContentSaver({ debounceMs, save }) returns a saver owning the 1.2s debounce, the pending dirty tracker (was rawPendingMarkdown), and flushNow({ keepalive }) / queue / cancel / clearPending / dirty. The actual PATCH + all reactive item/saveStatus/editorStore bookkeeping stays in the page as the injected save callback, so the module has no Svelte/API deps and is testable in plain node.

Rewired in [collection]/[slug]/+page.svelte:

• handleRawContentUpdate → rawContentSaver.queue(markdown)
• beforeunload keepalive block (BUG-2024) → rawContentSaver.flushNow({ keepalive: true }) — keepalive-on-unload, debounce-cancel, dirty prompt, and clear-on-save all preserved; collab flush still fires first.
• flushRawIfPending drain loop → uses saver.pending / saver.cancel() / saver.clearPending() (its own awaited PATCH loop unchanged).
• item-swap reset + the legacy non-collab handleContentUpdate path cancel the saver too, preserving the shared-timer mode-toggle non-trample.

pending is a plain closure var (not $state) per CONVE-1688 — the reactive dirty flag stays in editorStore.

B. progressMerge.ts — deduped progress fetch/merge (collection page)

New web/src/lib/collections/progressMerge.ts — pure plansProgressToMap + mergeChildAndCheckboxProgress (BUG-1509 preference logic) plus fetchCollectionProgress (parallel fetch + merge, per-fetch .catch(() => [])). Both duplicated copies in [collection]/+page.svelte (refreshProgress and inline in loadCollection) now call the shared fn; each keeps its own seq-guard / label / error handling so behavior is identical.

Tests

• contentSaver.test.ts (7, fake timers): debounce coalescing, dirty set/clear, flush-now cancels pending debounce, keepalive flag plumbed, no-save-when-clean, cancel leaves dirty, default debounce.
• progressMerge.test.ts (8): plans map, all three merge branches, defensive orphan-checkbox, empties.
• Full suite: 153 pass. npm run check: 0 errors. npm run build: success. Codex review: CLEAN.

Line deltas

• [collection]/+page.svelte: 2957 → 2891 (-66)
• [collection]/[slug]/+page.svelte: 4648 → ~4660 (raw-save mechanics moved to the tested module; the page-coupled PATCH body remains as the injected callback, so net LOC on this file is flat — the win is the extracted, tested primitive + removed unload duplication)

Behavior-preserving: no user-visible change to save timing, keepalive-on-unload (BUG-2024), dirty prompt, collab flush order, or progress badges.