fix(desktop): emit singular mention feed category so alerts route correctly

[morgmart]

Problem

Mentions — and thread replies that @-mention you — played the Needs action sound instead of the @Mentions sound. Reported by @morgmart: "I set Needs action to a different sound and it's the only one I ever hear."

Root cause

Two vocabularies got conflated in #475:

• Section / filter vocabulary (plural): mentions, needs_action, activity, agent_activity — the shape of FeedSections, the --types filter, and the agent-facing CLI docs.
• Per-item category vocabulary (singular for mention): mention, needs_action, activity, agent_activity — the FeedItemCategory contract in desktop/src/shared/api/types.ts, unchanged since Add desktop Home feed #12.

The Tauri feed builder reused the filter string "mentions" as each mention item's category. Only one word differs between the vocabularies, so only mentions broke. Every frontend consumer compares against the singular, so real mentions never matched and fell through to the resolver's needs_action fallback.

The E2E mock bridge emits the singular form, so tests never saw the drift.

Symptoms this fixes (all from the one mislabel)

• Mentions and mentioning thread replies played the Needs-action sound
• Mention notifications used the Needs-action title format
• Mentions in muted channels were suppressed (the mute-bypass never fired)
• Inbox / Home feed labelled mentions "Channel update"
• Channel activity popover's mentions list was always empty

Fix

Fix the owner, not the symptoms. FeedItemInfo.category becomes a FeedItemCategory enum whose serde form is exactly the TS union, so a misspelled category can't compile at the producer. A serialization test pins each variant to its wire string.

Frontend: slotForFeedKind maps every known category explicitly. The needs_action fallback for unknown categories is kept on purpose — a contract drift should cost the user the wrong sound, not a missed alert — but it now console.warns so the drift is visible to developers instead of masquerading as intended behavior. e2eBridge.ts and tauri.ts now derive the category type from types.ts instead of retyping it.

Not touched: the plural --types filter and FeedSections keys. Those are the section vocabulary and are correct as-is.

Verification

• just ci green (file-size ratchet, Rust/Tauri/desktop/mobile tests, desktop + web builds)
• New tests: 2 Rust (feed_item_category_serializes_to_frontend_contract, feed_item_from_event_carries_singular_mention_category), 3 TS in sound.test.mjs incl. one that feeds the old "mentions" string and asserts fallback + warning
• Runtime, dev build against the production relay: controlled test from an agent identity into a test channel —
  • mention in channel → @Mentions sound, inbox shows "Mentioned in" ✅ (was Needs-action)
  • thread reply with mention → @Mentions sound, once ✅ (was Needs-action)
  • plain thread reply in the channel being viewed → silent, as designed ✅

Reviewers

• @tlongwell-block — Sprout speaks Nostr. Nothing else. #475 introduced the plural category; please confirm it wasn't intentional
• @wesbillman — owner of the original FeedItemCategory contract (Add desktop Home feed #12) and most of the feed builder
• @taylorkmho — owner of the sound-slot model and resolver (feat(desktop): per-event notification sounds and alert controls #968); the fallback-with-warning shape is the part to weigh in on
• cc @klopez4212