feat(update): tell sideloaded installs that a newer version exists (#179)

AGE-110: 64% of 30d-active users sit on v0.4.10 and 0.2% on the newest
build, so a client-side fix (the AGE-105 Sentry noise gate) reaches almost
nobody. Play hygiene fixes one channel; the direct APK, the self-hosted
F-Droid repo and third-party mirrors have no update mechanism at all — a
device installed from a downloaded APK has literally no way to learn a newer
version exists.

- src/lib/update-check-policy.ts: pure, node --test'able decision logic —
  numeric version compare (a string compare puts 0.4.10 BEFORE 0.4.9, i.e.
  it would have told the largest stale cohort it was current), a 24h check
  throttle that survives a backwards clock, per-version dismissal, and a
  cached last-known-latest so the affordance survives between checks
- src/lib/update-check.ts: Android-only runtime wiring. One unauthenticated
  GET per 24h to the GitHub releases API (every non-Play channel is
  downstream of a GitHub release; expo-updates cannot replace a native
  binary, which is what this cohort needs). Never throws.
- UpdateBanner on the sessions list: one dismissible strip, no modal.
  "Not now" sticks for that version only.
- Settings "Version" row showed a hard-coded "1.0.0" for every build ever
  shipped. It now shows the real version, plus "0.4.10 -> 0.4.14" when an
  update exists (ignoreDismissed: dismissal silences the banner, not the
  place a user goes to check).
- en/zh-Hans strings, catalog parity kept.

Tests: 19 new cases in update-check-policy.test.ts; full suite 300 pass,
tsc --noEmit clean.

Co-authored-by: engineer <engineer@macbookpro.lan>
Co-authored-by: Paperclip <noreply@paperclip.ing>

app/(tabs)/index.tsx

@@ -27,6 +27,7 @@ import type BottomSheet from "@gorhom/bottom-sheet"
 import type { Session, Project } from "../../src/lib/sdk"
 import { DirectorySwitcher, DirectoryBrowserSheet } from "../../src/components/chat"
 import { groupByDirectory } from "../../src/lib/session-grouping"
+import { UpdateBanner } from "../../src/components/UpdateBanner"
 import { nameOf } from "../../src/lib/path-utils"
 import { SETUP_GUIDE_URL } from "../../src/lib/links"
 
@@ -548,6 +549,8 @@ export default function SessionsScreen() {
         </View>
       )}
 
+      <UpdateBanner isDark={isDark} />
+
       <FlatList
         data={rows}
         keyExtractor={(row) => (row.type === "header" ? `dir:${row.directory}` : row.session.id)}