feat: add bounded generative UI registry

[SebastianBoehler]

Summary

• Adds a typed generative UI registry for next_action, schedule_summary, route_hint, deadline_card, and quick_reply payloads.
• Validates payload type, required fields, and per-component arguments before rendering.
• Adds a Home preview section backed by static fixture payloads so the team can switch through generated component examples without an LLM.

Stacked after #56. Closes #49

Validation

• git diff --check
• flutter test test/generative_ui_registry_test.dart test/navigation_test.dart could not run in this runtime because flutter is not installed (command not found).

[SebastianBoehler]

Post-merge review pass: the intent looks solid for #49: a bounded component registry, fixture-backed preview, and tests around supported/unsupported component kinds. The main risk area I would tighten next is schema strictness for model-produced JSON.

One concrete follow-up: GenerativeUiRegistry._string currently uses value?.toString().trim(), so non-string values such as 123, true, or dates/objects with a string representation can pass fields that are reported as string-only. That weakens the “strict schemas” gate from #49 and could let malformed LLM payloads render as valid components instead of failing explicitly. Smallest fix: make _string return non-null only when value is String && value.trim().isNotEmpty, then add a focused test for numeric/bool type, title, body, and required arguments.

Other useful review angles for the next iteration:

• Wire this registry to the actual assistant/action layer only after generated actions are mapped through existing app permissions/navigation, not directly from payload arguments.
• Keep fixture payloads minimal and non-sensitive; they are currently static and safe.
• Run flutter test test/generative_ui_registry_test.dart test/navigation_test.dart in a Flutter-enabled environment, since this runtime still cannot execute Flutter tests.

Concrete next step: open a small follow-up PR that makes string validation type-strict and adds the malformed-value tests.