feat(ui): add glass theme with native macOS vibrancy blur-through

[jeffa-block]

Summary

Adds a Glass theme option that reveals the native macOS vibrancy blur layer already enabled in main.ts (vibrancy: 'window').

Implements #8008

The key insight

Electron's vibrancy: 'window' is already set on line 614 of main.ts — the native NSVisualEffectView frosted blur layer is running right now, just hidden behind solid CSS backgrounds. This PR makes those backgrounds semi-transparent so the blur shows through.

What changed

File	Change
theme-tokens.ts	New glassColorTokens — rgba() backgrounds, opaque text for readability
ThemeContext.tsx	Extended ThemePreference type to include 'glass'
ThemeSelector.tsx	Added Glass button (Layers icon from lucide)
main.css	.glass class with backdrop-filter, sidebar aliases, scrollbar styles
index.html	Handle glass in pre-paint theme initialiser to prevent flash

Design decisions

• Glass uses dark-mode text conventions — white text on translucent dark backgrounds reads well against any desktop wallpaper
• Borders are semi-transparent white (rgba(255,255,255,0.1)) — creates subtle frosted-glass panel edges
• Text stays fully opaque — readability over aesthetics
• No new Electron config — the native vibrancy was already there

Hardcoded colour audit

The codebase is clean — 343 background usages go through the token system and become transparent automatically. Only 2 hardcoded bg-[#hex] exist (AlertBox error/warning banners), and those should stay solid.

Platform behaviour

Platform	Glass theme result	Native blur	Notes
macOS	✅ Full frosted glass	vibrancy: 'window' + transparent: true already set	NSVisualEffectView — every Mac that runs Goose supports this
Windows	Dark translucent, no desktop blur	transparent: false, no setBackgroundMaterial()	rgba() backgrounds render against solid window. Readable, just not glassy. Windows Acrylic (setBackgroundMaterial('acrylic')) via Electron 30+ could enable native blur — separate scope.
Linux	Dark translucent, CSS blur only	No native vibrancy	backdrop-filter blurs app content, not desktop. Same as Windows visually.

macOS is the full experience. Windows and Linux get a darker, moodier variant of dark mode — functional and readable, just without the desktop showing through. Native blur on those platforms is separate scope with its own platform quirks.

Visual references

• Apple HIG — Materials — the design language this follows
• Apple NSVisualEffectView — what Electron's vibrancy wraps
• Microsoft Fluent Acrylic — the Windows equivalent (future scope)
• Warp terminal — shipping Electron app using native vibrancy
• Hyper terminal — another Electron app with vibrancy
• Electron BrowserWindow docs — vibrancy and setBackgroundMaterial options

Screenshots

Draft PR — screenshots to follow once tested locally.

[DOsinga]

I don't know. our settings & config is complicated enough as it is. I know @spencrmartin has plans to make colors more configurable, let's wait for that.