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/AppShell.vue

@@ -7,6 +7,7 @@ import { useLabelsStore } from "../stores/labels";
 import { useUiStore } from "../stores/ui";
 import CommandPalette from "./CommandPalette.vue";
 import Icon from "./Icon.vue";
+import ImportNotes from "./ImportNotes.vue";
 import LabelsModal from "./LabelsModal.vue";
 import { NOTE_SWATCH_CLASSES, type NoteColor } from "../notes/colors";
 
@@ -309,6 +310,7 @@ async function signOut() {
           <a href="/api/notes/export" download class="nav-link" title="Download all your notes as a zip">
             <Icon name="download" /> Export
           </a>
+          <ImportNotes />
         </nav>
       </aside>