fix: prevent TUI crash when opening gallery item in browser

[cursor]

Summary

• Fixes a critical crash in Gallery Detail when pressing [o] to open a generation in the browser.
• Adds a small openInBrowser helper in src/core/browser.ts and exports it via src/core/index.ts.
• Updates GalleryDetail to handle browser-open failures with an awaited promise + .catch(...) and display an in-app error instead of crashing.
• Adds src/core/browser.test.ts to validate successful opener invocation and failure propagation.

Bug / impact

On systems where launching a browser fails (common in headless terminals, CI shells, or misconfigured desktop environments), pressing [o] in gallery detail caused an unhandled promise rejection. In current Node runtimes this terminates the process, abruptly killing the TUI session and interrupting user workflow.

Root cause

open() was called inside an Ink input handler without awaiting or catching the returned promise. Rejections escaped the handler and became process-level unhandled rejections.

Validation

• pnpm typecheck ✅
• pnpm build ✅
• pnpm test ✅ (31 tests, including new browser helper tests)

Scope

Minimal, high-confidence fix focused only on the crash path; no broad refactors.