feat: sync ETag concurrency + fix chunk-transfer hang + sync overlay + auto-VACUUM

Sync optimistic concurrency (ETag/If-Match)
- webdav GET captures the response ETag; PUT sends it back as If-Match so
  the server rejects (412) our write when another device changed the file
  between our pull and push. A 412 re-runs the whole pull→merge→push
  (bounded to 3) so the other device's changes are folded in instead of
  clobbered. Servers without ETags → empty etag → no If-Match → falls
  back to last-write-wins (no regression). onWebdavResult gained a 4th
  etag arg.

Chunked webdav PUT (big vaults no longer black-screen on sync)
- The whole encrypted snapshot base64'd into a single cmd://webdav/put URL
  blew past WebView2's cap → black screen once the vault grew (20MB of
  attachments). PUT bodies now stream through the file/chunk transport and
  commit via a new webdav/put-commit (reads the accumulated buffer).

Chunk-transfer hang fix (root cause of the stuck "Preparing…" sync)
- All chunked transfers (saveFile/writeFile/webdav PUT) share one
  reqId-keyed resolver. A resolved chunk's stale 30s timeout would later
  delete the CURRENT chunk's resolver and fire the wrong res(), leaving
  that chunk's await pending forever. Extracted a single _streamChunks()
  helper whose ack CLEARS the pending timeout, so resolvers stay strictly
  one-at-a-time. Also fixed _webdavCall referencing the Bridge-local cmd()
  from module scope (latent ReferenceError).

Sync busy overlay
- syncStatus() now drives the global busy overlay too, so a running sync
  blocks stray clicks (e.g. the auto-backup "Choose…" picker) and reads
  like the manual backup. The account-mismatch confirm hideBusy()s first
  so it's visible above the overlay.

Auto-VACUUM (reclaim space after deleting large attachments)
- SQLite never shrinks the file on DELETE, so deleting big attachments
  left vault.db bloated (35MB for 11 tiny entries). DB.CompactIfBloated
  VACUUMs when >20% of pages are free AND >~2MB is reclaimable — called on
  startup and after each attachment delete. A healthy small vault pays
  nothing. (Verified: 35MB → 695KB after the deletes.)

Rebuild: BuildAssets + F9 (UMainForm + PM.Database + PM.Handler.Attachments).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CLAUDE.md

@@ -499,8 +499,14 @@ device, never transmitted.
    restore attachments ; both sides have it = compare `updated_at`,
    PUT if remote newer.
 7. `loadEntries()` + `buildSyncSnapshot()` for the post-merge state.
-8. `webdav/put` push the merged snapshot.
-9. Toast `X added · Y updated · Z deleted`.
+8. `webdav/put` push the merged snapshot, with `If-Match: <etag>` where
+   the etag was captured from the step-2 pull (optimistic concurrency).
+   A **412** means another device changed the file between our pull and
+   push → re-run the whole pull→merge→push (`runSyncNow(_attempt+1)`,
+   bounded to 3) so their changes are folded in instead of clobbered.
+   Servers without ETag support return an empty etag → no If-Match sent
+   → falls back to last-write-wins (same as before).
+9. Toast `pulled X new · Y updated · Z deleted · pushed N entries`.
 
 Sensitive actions (export, change master pw, recovery code…) still
 require master pw via `askReauth` — sync never substitutes.