vc=59 cont: wire bg subs refresh + R8 keep + Settings UI

Bundling the background-refresh worker (originally planned as
vc=60) into the same release as cache controls — they're both
storage-and-refresh user-facing knobs, ships cleaner together.

- StrawApp.onCreate calls FeedRefreshScheduler.applyFromSettings
- R8 keep rule for FeedRefreshWorker (same reason as
  UpdateCheckWorker — WorkManager instantiates via reflection)
- Settings UI: 'Auto-refresh subs' toggle (default off) +
  interval chip-row (30min / 1h / 6h) shown when enabled. Lives
  in the existing Local cache section since it's the same
  storage-and-refresh theme.

Worker calls uniffi.strawcore.subscriptionFeed which fans out 50
parallel RSS fetches in Rust — 50 subs refreshes in ~1-2s in the
background. Writes per-channel into FeedCacheStore so next cold
open of Subs paints instantly.

strawApp/proguard-rules.pro

@@ -86,4 +86,5 @@
 # renames our UpdateCheckWorker the scheduler enqueues it but the
 # instantiation fails silently and no checks ever run.
 -keep class com.sulkta.straw.feature.update.UpdateCheckWorker { *; }
+-keep class com.sulkta.straw.feature.feed.FeedRefreshWorker { *; }
 -keep class * extends androidx.work.ListenableWorker { *; }