feat: add Git diff preload toggle in Settings and respect it in diff loading

[Dimillian]

Motivation

• Large preloaded git diffs can freeze the app even when the user isn't viewing them, so we need a user-facing toggle to disable background diff preloading while keeping the default behavior on.

Description

• Add a Preload git diffs settings section and navigation entry in the settings UI with the subtitle Make viewing git diff faster, and a toggle wired to appSettings.preloadGitDiffs (frontend: src/features/settings/components/SettingsView.tsx).
• Introduce preloadGitDiffs in the shared AppSettings types and defaults on the frontend and backend (src/types.ts, src/features/settings/hooks/useAppSettings.ts, and src-tauri/src/types.rs) and persist it via existing settings storage.
• Wire the setting into the git controller: useGitPanelController now accepts gitDiffPreloadEnabled and gates background preload/loading logic so diffs are only preloaded when enabled or loaded on-demand when a file is explicitly selected (src/features/app/hooks/useGitPanelController.ts, src/App.tsx).
• Update affected tests and test fixtures to include the new default setting (src/features/settings/components/SettingsView.test.tsx).

Testing

• Ran npm run lint, which completed successfully.
• Ran npm run test (Vitest), which passed: 52 test files, 259 tests total (all passed).
• Ran npm run typecheck (tsc --noEmit), which completed successfully.
• Ran cargo check in src-tauri to validate Rust changes; it failed in this environment due to a missing system dependency (glib-2.0 / pkg-config), so Rust build validation is blocked by the environment rather than code errors.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

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