Add query bookmarks and saved queries library

[muk2]

Summary

Implements Issue #18 — a query bookmark system for saving, browsing, and loading frequently used SQL queries.

• Ctrl+Shift+S saves the current editor content as a bookmark with name, description, and optional tags
• Ctrl+Shift+O opens a searchable picker to browse and load saved queries
• Search filters by name, description, tags, and query content
• Ships with 6 built-in PostgreSQL admin snippets: table sizes, running queries, index usage stats, unused indexes, lock monitoring, cache hit ratio
• User bookmarks persist to ~/.config/pgrsql/bookmarks.json
• Delete key removes user bookmarks (built-in snippets cannot be deleted)

Changes

File	Changes
src/bookmarks.rs	New module: SavedQuery struct, load_bookmarks(), save_bookmarks(), built_in_snippets(), 6 unit tests
src/main.rs	Added mod bookmarks
src/ui/app.rs	Added Focus::BookmarkPicker/BookmarkSave, BookmarkPickerState, BookmarkSaveState, filtered_bookmarks(), picker and save input handlers, Ctrl+Shift+S/O keybindings
src/ui/components.rs	Added draw_bookmark_picker(), draw_bookmark_preview(), draw_bookmark_save(), updated help overlay

Test plan

• All 130 tests pass (124 existing + 6 new)
• cargo clippy clean
• cargo fmt clean
• Manual test: Ctrl+Shift+S opens save dialog, fill name/description/tags, Enter saves
• Manual test: Ctrl+Shift+O opens picker with built-in snippets
• Manual test: Type in search field to filter bookmarks
• Manual test: Enter loads selected bookmark into editor
• Manual test: Delete removes user bookmarks (not built-ins)

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