feat: add calculator provider and frecency tracking

Calculator:
- Type "= expression" or "calc expression" for instant math evaluation
- Supports standard math functions (sqrt, sin, cos, etc.) and constants (pi, e)
- Copies result to clipboard on Enter via wl-copy

Frecency:
- Firefox-style algorithm: score = launch_count * recency_weight
- Boosts frequently and recently launched items in search results
- Persists to ~/.local/share/owlry/frecency.json
- Configurable via providers.frecency and providers.frecency_weight

New config options:
- providers.calculator = true
- providers.frecency = true
- providers.frecency_weight = 0.3

UI updates:
- Added :calc prefix support
- Calculator badge with yellow styling
- Updated hints to show "= calc" syntax

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/data/mod.rs

@@ -0,0 +1,3 @@
+mod frecency;
+
+pub use frecency::FrecencyStore;