Import: ThoughtSync-native round-trip + Google Keep Takeout

Complete the export/import pair (task 1907). POST /api/notes/import takes
an uploaded .zip and appends its notes — never overwriting existing ones.

Two formats, auto-detected:
- ThoughtSync export: recognized by its notes.json (app == thoughtsync);
  round-trips title/body/color/kind/pinned/archived/remind_at/timestamps/
  labels/items and re-attaches image media from the zip.
- Google Keep Takeout: each Keep <note>.json → a note. Maps title,
  textContent/listContent (+ checked), labels, Keep color enum (nearest
  palette match), isPinned/isArchived, isTrashed (→ trash), created/edited
  microsecond timestamps; folds annotation URLs into the body; resolves
  attachment filePaths relative to the note's folder.

Imported notes reuse create_note's derivation + reconciliation:
display-title derive, #tag reconcile, [[wiki-link]] rewrite. Explicit
labels attach as manual (via_tag=false); inline #tags reconcile as tags.
Image attachments copied into media storage; non-image types (e.g. Keep
audio) skipped until any-file attachments land.

Frontend: an Import control in the sidebar (next to Export) — hidden file
input + FormData POST + result toast ("Imported N notes (M skipped)"),
reloading the board + labels. New upload icon; notes-store importNotes().

Tests: import auth-guard + pure-helper coverage (_usec_to_dt, _keep_spec
list/text/color/annotation/attachment mapping, _native_spec round-trip).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/Icon.vue

@@ -24,6 +24,7 @@ const paths: Record<string, string> = {
   merge: '<circle cx="18" cy="18" r="3"/><circle cx="6" cy="6" r="3"/><path d="M6 21V9a9 9 0 0 0 9 9"/>',
   history: '<path d="M3 12a9 9 0 1 0 9-9 9.75 9.75 0 0 0-6.74 2.74L3 8"/><path d="M3 3v5h5"/><path d="M12 7v5l4 2"/>',
   download: '<path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"/><polyline points="7 10 12 15 17 10"/><line x1="12" x2="12" y1="15" y2="3"/>',
+  upload: '<path d="M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4"/><polyline points="17 8 12 3 7 8"/><line x1="12" x2="12" y1="3" y2="15"/>',
 };
 </script>