Search index: thread active-letter id into IndexableMessage.letter_id

[intendednull]

Follow-up to #355.

crates/web/src/app.rs builds IndexableMessage from the messages signal and currently hard-codes letter_id: None because there is no active-letter signal in crates/web/src/state.rs to read from. Letters/DMs is specced in docs/specs/2026-04-19-ui-design/letters-dms.md but the feature has not landed:

• No letter_id / current_letter signal in AppState.
• command_palette + tab_bar reference letters as a feature-flagged future surface.
• SearchScope::ThisLetter(id) and SearchScope::AllLetters exist in the executor and rely on Posting.letter_id, but no code path produces a non-None letter_id today.

When letters-dms.md ships:

1. Add current_letter: ReadSignal<Option<String>> (or equivalent) to AppState.
2. Subscribe to it in the search-index Effect in crates/web/src/app.rs alongside current_channel / active_server.
3. Set letter_id: current_letter.get() (or per-message lookup if the messages signal mixes channels and letters).
4. Add a client-tier test confirming SearchScope::ThisLetter returns the expected hits.

Unblocks the documented in:#letter / scope-flip operator behaviour. Severity: medium (UX gap, not security).

[intendednull]

Duplicate of #441 — same problem (letter_id: None hard-coded in indexer effect, blocked on letters-dms feature). Consolidate work under #441.

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