Make mail navigation items linkable

[keilogic]

Summary

• Render the Compose, Drafts, and Overview mail navigation actions as router-backed anchors instead of button-only controls.
• Preserve the existing mobile side-menu close behavior for normal app-handled clicks.
• Leave modified/middle-click browser navigation alone so users can open these entries in a new tab or window.

Current master already renders folder rows, including Inbox and user folders, as anchors. This fills the remaining button-only mail navigation entries mentioned in #238.

Tests

• npm ci
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npm run lint -- --quiet
• npx ng test --include src/app/app.component.spec.ts --watch=false --browsers=FirefoxHeadless
• PowerShell equivalent of npm run build with SKIP_CHANGELOG=1: node src/build/pre-build.js; npx ng build runbox7 --configuration production --base-href=/app/; node src/build/post-build.js (build hash 93a5932466006d9c; existing Angular warnings only)
• npm run policy (passes; reports existing historical commit-message warnings)

AI disclosure

This PR was implemented with assistance from OpenAI Codex/GPT-5 in src/app/app.component.html, src/app/app.component.ts, and src/app/app.component.spec.ts. I reviewed the changes and ran the checks above.

Fixes #238.