feat: add proactive home feed snapshot

[SebastianBoehler]

Summary

• Adds typed HomeFeedSnapshot models for daily summary, next action, urgent items, and source freshness.
• Renders the proactive feed at the top of Home from cached/local state without blocking app entry.
• Updates feed and navigation tests for the new snapshot contract.

Closes #48

Validation

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

[SebastianBoehler]

Post-merge review note: the vertical slice looks aligned with #48 and the merged CI run is green, so I would keep the follow-up small.

Smallest useful next action:

• Make NextAction genuinely actionable, or rename the label field so it is not presented as a call to action.

Why this is worth a follow-up:

• HomeFeedSnapshot emits labels such as Open settings, Open schedule, Open notes, and Open chat in flutter_app/lib/src/feed_summary.dart.
• ProactiveFeedSection currently only renders an actual button when action.label == 'Refresh'; all other next-action labels are effectively passive text in the tile.
• That leaves the first-screen feed answering “what should I care about next?” but not reliably letting the user take the suggested action from the feed itself, which is one of the product-risk areas for this vertical slice.

Useful review angles for the follow-up:

• Keep the feed snapshot model deterministic and provider-independent.
• Avoid label-string branching for behavior; prefer an enum/target/action type on NextAction and route from HomeView or the router boundary.
• Add a widget/navigation test for at least one non-refresh next action, e.g. Open schedule or Open notes.

Validation checked here:

• git diff --check 9a4dd63..9c36049 -- flutter_app passed locally.
• GitHub Actions for the merged PR passed formatting, analysis, Flutter tests, and platform builds.
• I could not run Flutter tests locally in this runtime because flutter is not installed.