feat(themes): add built-in theme support for overlay

[hikaps]

Summary

Adds theming support to the overlay (issue #54, v1: built-in themes). All hardcoded overlay colors are extracted into a semantic brush-key ResourceDictionary, and four built-in themes are selectable in settings.

Changes

• Themes/Default.xaml — the current dark appearance, as semantic brush keys
• Themes/Light.xaml — light surfaces/dark text
• Themes/Minimal.xaml — near-monochrome with muted accents
• Themes/Vibrant.xaml — saturated cyan/purple/red accents on blue-tinted dark
• Themes/ThemeManager.cs — resolves theme names (case-insensitive, fallback to Default), loads compiled dictionaries via pack URI, applies to window before InitializeComponent, resolves brushes for code-behind use
• OverlayWindow.xaml — ~77 inline color sites converted to {DynamicResource} references (surfaces, text tiers, focus borders, all three accent families with hover/pressed variants)
• OverlayWindow.xaml.cs — theme applied in constructor; hardcoded brushes (section focus highlight, achievement gold/muted text, shortcut button factory) replaced with themed lookups; new themeName constructor param
• OverlaySettings.cs / OverlaySettingsView.xaml / OverlaySettingsViewModel.cs — ThemeName setting, Appearance group box with theme combo, validation against built-in theme list
• OverlayService.cs — passes the selected theme into the window
• tests/ThemeManagerTests.cs — name resolution, XAML key coverage vs Default, key-set consistency across all themes

Design

• 34 semantic brush keys (surfaces, text tiers, accents + hover/pressed variants). DynamicResource (not StaticResource) so themes can be swapped at runtime without re-creating the window.
• Theme dictionaries are compiled Pages in the OverlayPlugin assembly; loaded via /OverlayPlugin;component/Themes/{Name}.xaml.
• Unknown/empty theme names fall back to Default; settings validation prevents persisting an invalid name.

Testing

• Unit tests: ResolveThemeName normalization, every {DynamicResource} key in OverlayWindow.xaml exists in Default, all four theme dictionaries define the identical key set.
• Manual (Windows): open settings → Appearance → pick each theme, open the overlay, verify colors change and no XAML resource errors in the log.

Closes #54