feat: add terminal link browser preference

[Yt4aZaveta]

Add a three-state "Open links" setting to Settings → Controls for links from terminals and agent output: CanvasTTY, External browser, or Ask. Previously every click opened the browser chooser; Ask preserves that behavior and remains the default.

Persist and validate the preference, migrate existing settings to Ask, and route clicks through the existing URL validation and browser APIs. Include Russian and English labels and settings persistence coverage.

Validation:

• 85 targeted tests passed for terminal links, link targets, settings normalization/persistence, and settings UI.
• Typecheck, build, and git diff --check passed.
• The user manually tested the GUI and reported that the switches work correctly.
• Source and built-bundle secret scans passed after excluding the gitignored .tmp GUI packaging artifact.
• The full npm test run was not green: failures included EPERM on temporary paths, Unix socket paths exceeding the limit in this worktree, and a git-audit fixture failure. The full suite still needs confirmation in CI.

Review found no blocking defects in the changed production code. The commit contains only five source files and two test files; build output and temporary artifacts are excluded.