feat: add inline folder creation to inbox filing dropdown

[h4yfans]

What

Add the ability to create new folders directly from the inbox filing dropdown, eliminating the need to leave the filing flow.

Why

When filing inbox items, users could only select existing folders. Creating a new folder required navigating away from the inbox, breaking the filing flow. This is a common friction point when organizing new content.

How

• Added search-to-create pattern in the filing section popover: when the search query matches no existing folders, a tint-highlighted "Create" option appears
• Enter key or click creates the folder via existing createFolder IPC, invalidates the react-query cache, selects the new folder, and closes the popover
• Supports nested paths via slash syntax (e.g. Research/AI) — backend ensureDirectory handles recursive creation
• Added subfolder hint below search input for discoverability
• No backend changes needed — reuses existing CREATE_FOLDER IPC handler

Type

• feat — new feature
• fix — bug fix
• refactor — restructure without behavior change
• style — visual/UI only
• perf — performance improvement
• test — adding or updating tests
• chore — tooling, deps, config
• docs — documentation only
• ci — CI/CD changes

Test plan

• Unit tests added/updated
• Integration tests added/updated
• Manual testing (describe below)

1. Open inbox, select an item
2. Open the folder dropdown in the filing section
3. Type a non-existing folder name (e.g. "recipes")
4. Verify "Create 'recipes'" option appears with tint styling
5. Click or press Enter — folder is created and selected, popover closes
6. Re-open dropdown — new folder appears in "All folders"
7. Test nested path: type "recipes/italian" — verify creation works
8. Test edge cases: trailing slash stripped, empty search shows no create option

Checklist

• Self-reviewed the diff
• No hardcoded secrets or credentials
• Files stay under ~500 LOC
• Follows immutable data patterns