Reintroduce mail and experimental GenUI cards

[SebastianBoehler]

Summary

Reintroduces the mail surface and expands the assistant's generative UI path. Some GenUI pieces are intentionally experimental and should get careful review before merge.

Changes

• Restores the mail UI/repository flow with inbox loading, message details, mailbox switching, unread filtering, search, pull-to-refresh, and short-lived repository caching.
• Adds richer generated UI card support for mail, deadlines, talks, academic status, study progress, Mensa, campus locations, schedules, quick replies, next actions, and custom views.
• Renders tool-backed cards in chat and trims duplicate prose when the card already displays the fetched data.
• Moves local LiteRT tool routing into Dart, keeps stable local system instructions separate from volatile context, and adds generation timeouts/model unload behavior.
• Adds inline math Markdown support for assistant messages.
• Extends tests for mail caching/view behavior, GenUI payload builders, and local prompt/tool-loop behavior.

Notes

• GenUI rendering and custom view payloads are still exploratory; review should focus on payload validation, fallback behavior, and UX failure modes.
• Mail access still depends on the user's locally stored profile/password flow.

Validation

• Not run in this PR creation step. Diff inspected against origin/main before opening the PR.