feat(search): add configurable global search exclusions in settings

[Rizmi]

Description

Allow users to configure custom search exclusions in Settings under "SEARCH & FILTERING". Exclusions support both folder names (e.g. .venv, matching any folder with that name across the directory tree) and specific directory paths (e.g. ~/Downloads or /var/log, matching only that directory and its subtrees).

• Adds search_exclusions preference to Preferences and ThemeManager (Vec<String>, serialized only when non-empty).
• Implements SearchExclusions helper to classify entries into folder names vs directory paths (resolving ~ to user home).
• Updates search tree indexing (index_trees_with_exclusions, recursive walker scheduler, and single-directory walker) to skip excluded folder names anywhere in the hierarchy and specific directory paths.
• Provides a dedicated "Global search exclusions" modal dialog matching the layout, tokens, and style 1:1 with the "Copy to" dialog (modal_layout, entry with inline error validation, "Browse…" button via gtk::FileDialog, type badges for folder name vs directory, and delete action for each row).
• Wires the "Manage" row under "SEARCH & FILTERING" in General settings and indexes the action in settings search (Target { id: "search-exclusions", title: "Global search exclusions", ... }).
• Adds unit and integration tests covering configuration persistence, exclusion classification, directory pruning in search results, and dialog validation.

Note

In-folder filtering (search in current pane) and destination selection dialogs intentionally operate within their explicit browsing context and bypass global search exclusions (documented in search.rs).

Visual evidence

screenrecording-2026-09-16_11-07-05.mp4

How to test

1. Open Settings (Ctrl+,) and navigate to General -> SEARCH & FILTERING.
2. Click Manage on the Global search exclusions row: verify the modal dialog opens cleanly over the blurred settings overlay.
3. Test adding a folder name:
   • Type .venv or node_modules and press Enter (or click "Add").
   • Verify it appears in the list with a "Folder name" badge.
4. Test adding a directory path:
   • Type ~/Documents/Private or use the "Browse…" button to select a directory.
   • Verify it appears in the list with a "Directory" badge.
5. Test validation:
   • Attempting to add an empty name, root/home (/ or ~), ambiguous relative paths without leading / or ~/, or duplicate entries shows an inline error message and highlights the field with form-field-error.
6. Test search exclusion behavior:
   • In global search (Ctrl+K), search for a file known to be inside an excluded folder or directory.
   • Verify matching items inside excluded paths are not returned in results.
7. Test removal:
   • Click the "Remove" button next to an entry in the exclusions modal.
   • Verify the entry is removed immediately, persisted, and subsequent searches include that path again.

Related issue

Closes #1062