Add 3-way Transparency mode (glass/blur/reduced) and migrate legacy setting

[Dimillian]

Motivation

• Replace the old boolean "Reduce transparency" toggle with a mode-based TransparencyMode to support macOS 26 Liquid Glass (glass/blur/reduced) while keeping sensible fallbacks for Linux and older macOS.
• Preserve existing user intent by migrating the legacy reduceTransparency value to the new mode and default to glass where supported.

Description

• Introduced the TransparencyMode type and storage/key migration from the legacy reduceTransparency boolean to a new transparencyMode (glass|blur|reduced).
• Implemented useTransparencyPreference() to read legacy settings, persist transparencyMode in localStorage, detect isGlassSupported() and expose transparencyMode, setTransparencyMode, availableModes, and glassSupported.
• Updated useLiquidGlassEffect() to accept transparencyMode and apply the correct behavior: enable Liquid Glass for glass, use the platform Effect.HudWindow/blur for blur, and clear effects for reduced (also disabling Liquid Glass when leaving glass).
• Wired the new API through useAppSettingsController() and App.tsx, updated app class application to use transparencyMode === "reduced" for the legacy CSS hook.
• Reworked settings UI: replaced the single toggle with a segmented control showing available modes (Glass, Blur, Reduced) and added styles for the segmented control; updated unit tests to use the new props/behaviour.

Testing

• Ran npm run lint; completed with warnings (TypeScript version warning and an existing react-hooks/exhaustive-deps warning in GitDiffViewer.tsx).
• Ran npm run test (vitest run) and all tests passed.
• Ran npm run typecheck (tsc --noEmit) and type-checking passed with no errors.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

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