fix: vault cache misses files in new directories, breaking theme restore

Root cause: git_uncommitted_files() used `git status --porcelain` which
reports new untracked directories as `?? theme/` instead of listing
individual files inside. The .md filter skipped directory entries, so
files in newly-created directories (theme/default.md, etc.) were
invisible to the cache system.

Fix: additionally use `git ls-files --others --exclude-standard` which
lists individual untracked files, resolving directories to their
contents.

Also:
- Add type/theme.md definition (icon: palette) to restore and getting-started vault
- Seed theme/*.md vault notes in getting-started vault
- Fix mock create_vault_theme to add entries for dev mode

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

src/mock-tauri/mock-handlers.ts

@@ -319,6 +319,16 @@ line-height-base: 1.6
 
 # ${displayName}
 `
+    const now = Date.now() / 1000
+    MOCK_ENTRIES.push({
+      path, filename: `${slug}.md`, title: displayName, isA: 'Theme',
+      aliases: [], belongsTo: [], relatedTo: [], status: null, owner: null,
+      cadence: null, archived: false, trashed: false, trashedAt: null,
+      modifiedAt: now, createdAt: now, fileSize: 512, snippet: `A custom ${displayName} theme.`,
+      wordCount: 10, relationships: {}, icon: null, color: null, order: null,
+      sidebarLabel: null, template: null, sort: null, view: null,
+      outgoingLinks: [], properties: {},
+    })
     syncWindowContent()
     return path
   },