Add schema search and filter to sidebar

[muk2]

Summary

Adds a search/filter bar to the sidebar for quickly finding tables, views, and databases by name.

• Activation: Press / in the sidebar to activate filter input (Vim-style)
• Real-time filtering: Sidebar tree filters as user types, showing only matching items
• Substring matching: Case-insensitive substring match (e.g., "user" matches "users", "user_roles")
• Context preservation: Filtered results show parent schema context
• All tabs: Filter works across Databases, Tables, and History tabs
• Clear: Escape clears filter and returns to full tree view

Keyboard Shortcuts

Key	Action
/	Activate filter input
Esc	Clear filter / exit filter mode
Enter	Exit filter input (keep filter)
Backspace	Delete filter chars (deactivates on empty)

Files Changed

File	Description
src/ui/app.rs	Filter state, input handling, match logic, 3 unit tests
src/ui/components.rs	Filter bar rendering, filtered tree/list drawing

Test plan

• cargo build passes
• cargo test — 228 tests pass (3 new)
• / activates filter bar in sidebar
• Typing filters tables in real-time
• Esc clears filter
• Works across all sidebar tabs

Closes #37

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