fix: editable git prompt template in Git settings

[Dimillian]

Motivation

• Provide a way for users to customize the default PR prompt used when asking questions about GitHub pull requests from the Git panel.

Description

• Add a new gitPullRequestPrompt setting (frontend src/types.ts, backend src-tauri/src/types.rs) with a default template and normalization in useAppSettings.
• Expose an editable textarea in the Git section of Settings with save/reset controls and draft handling (SettingsGitSection.tsx, SettingsView.tsx).
• Extend the PR prompt generator to accept a template with tokens and fill them (src/utils/pullRequestPrompt.ts), and wire the configurable template into the PR composer flow (usePullRequestComposer.ts, App.tsx).
• Update tests and test fixtures to exercise template passing (usePullRequestComposer.test.tsx, SettingsView.test.tsx).

Testing

• Ran npm run lint and it completed successfully after fixes.
• Ran npm run test (Vitest) and the full test suite passed (439 passed).
• Ran npm run typecheck (tsc --noEmit) and it completed successfully.
• Ran cargo check in src-tauri which failed due to a missing system dependency (glib-2.0) in the environment and is unrelated to the added Rust serialization changes.

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Codex Task