Complete UI/UX & performance overhaul — cocoa brown theme (v2.1.0)

[Daolyap]

Summary

Full visual and performance overhaul of every window, as requested.

UI/UX

• New Themes/CocoaTheme.xaml — single source of truth: cocoa-brown palette (espresso surfaces + caramel accent), all control styles, and hand-drawn vector icon geometries. Replaces four copies of the per-window "glassmorphism" styles.
• All windows (Main, Editor, Settings, History) share consistent professional chrome: custom title bar, proper caption buttons (minimize/maximize/close with vector glyphs, red close hover per Windows convention).
• Every emoji glyph replaced with vector Path icons — emoji rendered inconsistently via font fallback and read as unpolished.
• Region selector now behaves like a proper snipping tool: dimmed surround with the selection cut out, caramel outline, live W × H size badge, and an instruction hint.
• Editor: zoom % indicator, keyboard tool shortcuts now move the active-tool highlight, themed text dialog with Enter/Esc support, caramel selection chrome.
• Settings: capture-history controls (toggle, retention 0–500, clear with confirmation, open folder) and a View Logs button — closes Opus-Speaks sections C & D. Also fixed the default-file-format combo, which silently never saved (read SelectedItem as string when items are ComboBoxItems).
• About dialog reads the assembly version instead of a hard-coded "2.0.0", and drops the "Incredible AI Slop" tagline.

Performance

• Migrated all windows off AllowsTransparency to WindowChrome (Opus-Speaks section B). Layered windows are composed in software for the entire surface — this was the single biggest source of editor lag, and the region selector paid it across every monitor on every capture.
• Removed every DropShadowEffect/BlurEffect (incl. a stray BlurEffect Radius=0) — WPF bitmap effects force per-element software rendering.
• Pixelate tool rewrite: previously rendered the entire screenshot into a RenderTargetBitmap (~33 MB at 4K) and allocated a CroppedBitmap per 10-px block. Now copies only the covered region once and block-averages in a single buffer. Faster and a large RAM saving.
• Frozen shared brushes; HighQuality bitmap scaling on the editor image to remove zoom-out shimmer artifacts.

Verification

• dotnet build clean, dotnet test 95/95 passing.
• Editor, Settings, and History windows rendered through an off-screen harness and visually inspected (cocoa theme, icons, layout all correct). MainWindow uses only theme keys proven by those windows (it can''t be harness-loaded because its constructor wires the tray icon + global hotkeys).
• Manual on-Windows checklist still recommended before release for: region capture drag, pixelate visual parity, maximize behaviour of WindowChrome windows.

Notes

• Version bumped to 2.1.0.
• CLAUDE.md updated with the theme/rendering rules so the slow patterns don''t come back.
• Known pre-existing issue (not addressed here, flagged for the audit branch): RegionSelector maps mouse DIPs to bitmap pixels 1:1, which is only correct at 100 % display scaling — the app manifest declares PerMonitorV2.

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