fix(desktop): normalize feed item category contract to singular mention (#2106)

[hmadhsan]

Fixes #2106

🐛 What was the issue?

Native get_feed (in desktop/src-tauri/src/commands/messages.rs) was emitting feed items with category: "mentions" (plural), while the declared TypeScript FeedItemCategory union value (types.ts) and every frontend consumer (feed.ts, inbox.ts) checked against singular "mention".
As a result, in native mode, mention feed items fell through to generic notification fallbacks ("Needs Action in #channel" instead of "@Mention") and missed mention-specific inbox classification/labeling.

🛠️ What did we fix?

• Tauri Native Backend: Updated get_feed in desktop/src-tauri/src/commands/messages.rs so feed_item_from_event receives singular "mention" instead of "mentions".
• Frontend API Boundary: Updated fromRawFeedItem in desktop/src/shared/api/tauri.ts to canonicalize wire "mentions" to "mention" as a defensive guard.
• Unit Tests: Added unit tests in desktop/src/shared/api/tauriFeed.test.mjs verifying category normalization.

🧪 Testing & Verification

• Added unit tests in tauriFeed.test.mjs asserting that fromRawFeedItem canonicalizes "mentions" to "mention" and preserves "mention".
• Verified category contract alignment across Rust models and TypeScript types.