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>

config.example.toml

@@ -32,6 +32,7 @@ border_radius = 12
 # accent = "#7aa2f7"
 # accent_bright = "#89b4fa"
 # badge_app = "#9ece6a"
+# badge_calc = "#e0af68"
 # badge_cmd = "#7aa2f7"
 # badge_dmenu = "#bb9af7"
 # badge_uuctl = "#f7768e"
@@ -40,3 +41,10 @@ border_radius = 12
 applications = true
 commands = true
 uuctl = true
+
+# Calculator provider (type "= 5+3" or "calc 5+3")
+calculator = true
+
+# Frecency: boost frequently/recently used items in search results
+frecency = true
+frecency_weight = 0.3  # 0.0 = disabled, 1.0 = strong boost