v1.0.28: fix sync rewrite/delete loop, Avast-inspired icon

Sync loop root-cause fixes (three independent bugs):

1. Folder change clears stale file states (AddPairViewModel): when
   localPath or remotePath changes on an existing pair, all
   SyncFileStateEntity records are wiped. Previously those stale records
   caused every sync to attempt DELETE_REMOTE on the old folder's files
   and to treat all new-folder files as changed — causing both the
   "deleting 32 files" loop and rewrites on every run.

2. Download stores null localModifiedAt (SyncEngine): SAF document
   cursors can return a stale mtime immediately after a write. Storing
   null forces the SKIP reconciliation pass on the next sync to read
   the actual walkFiles cursor value, breaking the download->changed->
   download loop caused by mtime inconsistency.

3. Second-precision mtime comparison (syncDecide): WebDAV RFC-1123 has
   1-second precision; FAT32 has 2-second precision. Comparing at
   millisecond level caused phantom "changed" detections after syncing
   to/from these systems. Now uses epochSecond for both local and remote.

Icon: three bold teal/red/yellow teardrop streaks (Avast palette) flying
into a white cloud centre, on dark charcoal background.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

version.properties

@@ -1,2 +1,2 @@
-VERSION_NAME=1.0.27
-VERSION_CODE=28
+VERSION_NAME=1.0.28
+VERSION_CODE=29