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>

delphi-backend/UMainForm.pas

@@ -1101,7 +1101,8 @@ begin
   // - PUT ok  → status=200/201/204, body=''
   // - test    → status=200..399 means reachable, body=''
   // Network errors → status=0, body=exception message.
-  else if (ACmd = 'webdav/get') or (ACmd = 'webdav/put') or (ACmd = 'webdav/test') then
+  else if (ACmd = 'webdav/get') or (ACmd = 'webdav/put') or (ACmd = 'webdav/test')
+       or (ACmd = 'webdav/put-commit') then
   begin
     var LMethod := ACmd;
     var LReqId  := GetParam('reqId');
@@ -1109,6 +1110,26 @@ begin
     var LUser   := GetParam('user');
     var LPwd    := GetParam('pwd');
     var LData   := GetParam('data');
+    // put-commit: the (large) body arrived in chunks via file/chunk,
+    // accumulated in FFileSaveChunks keyed by reqId. Pull it out and treat
+    // the rest as a normal PUT.
+    if ACmd = 'webdav/put-commit' then
+    begin
+      var LSB: TStringBuilder;
+      if FFileSaveChunks.TryGetValue(LReqId, LSB) then
+      begin
+        LData := LSB.ToString;
+        LSB.Free;
+        FFileSaveChunks.Remove(LReqId);
+      end
+      else
+        LData := '';
+      LMethod := 'webdav/put';
+    end;
+    // Optimistic concurrency: JS passes the ETag it saw at pull time; we
+    // send it as If-Match on the push so the server rejects (412) the
+    // write when another device changed the file in between.
+    var LIfMatch := GetParam('ifmatch');
     TThread.CreateAnonymousThread(
       procedure
       var
@@ -1120,10 +1141,12 @@ begin
         LBodyB64: string;
         LStatus: Integer;
         LErr: string;
+        LEtag: string;
       begin
         LStatus  := 0;
         LBodyB64 := '';
         LErr     := '';
+        LEtag    := '';
         try
           LHttp := System.Net.HttpClient.THTTPClient.Create;
           try
@@ -1141,6 +1164,7 @@ begin
               try
                 LResp := LHttp.Get(LUrl, LBodyStream);
                 LStatus := LResp.StatusCode;
+                LEtag := LResp.HeaderValue['ETag'];
                 if (LStatus >= 200) and (LStatus < 300) and (LBodyStream.Size > 0) then
                 begin
                   SetLength(LBytes, LBodyStream.Size);
@@ -1158,8 +1182,13 @@ begin
               LBytes := TNetEncoding.Base64.DecodeStringToBytes(LData);
               LReqStream := TBytesStream.Create(LBytes);
               try
-                LResp := LHttp.Put(LUrl, LReqStream);
+                if LIfMatch <> '' then
+                  LResp := LHttp.Put(LUrl, LReqStream, nil,
+                    [System.Net.URLClient.TNetHeader.Create('If-Match', LIfMatch)])
+                else
+                  LResp := LHttp.Put(LUrl, LReqStream);
                 LStatus := LResp.StatusCode;
+                LEtag := LResp.HeaderValue['ETag'];
               finally
                 LReqStream.Free;
               end;
@@ -1182,7 +1211,7 @@ begin
         TThread.Queue(nil,
           procedure
           var
-            LEscReq, LEscPayload: string;
+            LEscReq, LEscPayload, LEscEtag: string;
           begin
             LEscReq := StringReplace(LReqId, '"', '\"', [rfReplaceAll]);
             // GET success path → ship body base64. Otherwise the field
@@ -1196,12 +1225,16 @@ begin
             LEscPayload := StringReplace(LEscPayload, '"', '\"', [rfReplaceAll]);
             LEscPayload := StringReplace(LEscPayload, #13, '', [rfReplaceAll]);
             LEscPayload := StringReplace(LEscPayload, #10, '\n', [rfReplaceAll]);
+            LEscEtag := StringReplace(LEtag, '\', '\\', [rfReplaceAll]);
+            LEscEtag := StringReplace(LEscEtag, '"', '\"', [rfReplaceAll]);
+            LEscEtag := StringReplace(LEscEtag, #13, '', [rfReplaceAll]);
+            LEscEtag := StringReplace(LEscEtag, #10, '', [rfReplaceAll]);
             WebBrowser.ExecuteJavaScript(
               'if(window.Bridge&&Bridge.onWebdavResult)' +
               'Bridge.onWebdavResult("' + LEscReq + '",' +
-              IntToStr(LStatus) + ',"' + LEscPayload + '")');
-            LogLine(Format('%s %s → %d (%d bytes payload)',
-              [LMethod, LUrl, LStatus, Length(LEscPayload)]));
+              IntToStr(LStatus) + ',"' + LEscPayload + '","' + LEscEtag + '")');
+            LogLine(Format('%s %s → %d (%d bytes payload, etag=%s)',
+              [LMethod, LUrl, LStatus, Length(LEscPayload), LEtag]));
           end);
       end).Start;
   end