[tech-debt] Stale `#[allow(dead_code)]` + dead `save_download` helper (blocked on in-flight PRs)

[intendednull]

Context

Follow-up from the tech-debt audit that produced #194. Three cleanups
were deferred because the targeted files are under heavy modification
in PRs #185, #187, #188 and any edit would conflict:

1. crates/web/src/state.rs:169 — #[allow(dead_code)] on
   UiState::settings_tab. The field is read in app.rs (e.g.
   app_state.ui.settings_tab.get_untracked()) and written in four
   places. The annotation is stale and can be removed.

2. crates/web/src/state.rs:182 — #[allow(dead_code)] on
   VoiceState::voice_participants_map. Written from
   event_processing.rs and app.rs; reset in
   VoiceWriteSignals::reset(). Annotation is stale.

3. crates/client/src/storage.rs:391 — save_download function,
   native-only (#[cfg(not(target_arch = "wasm32"))]), no callers
   anywhere in the workspace. The comment notes it predates a legacy
   EventStore compat removal. Candidate for deletion.

Action

Land after #185 / #187 / #188 merge (or one of them, whichever edits
the above files last). Mechanical one-commit cleanup — no design
input needed. Feel free to roll into the next audit pass.