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-tauri/src/theme/mod.rs

@@ -10,7 +10,8 @@ use std::path::Path;
 pub use create::{create_theme, create_vault_theme};
 pub use defaults::*;
 pub use seed::{
-    ensure_vault_themes, restore_default_themes, seed_default_themes, seed_vault_themes,
+    ensure_theme_type_definition, ensure_vault_themes, restore_default_themes,
+    seed_default_themes, seed_vault_themes,
 };
 
 /// A theme file parsed from _themes/*.json in the vault.