fix(dashboard): synchronize mobile pairing state - #7471

Substantive changes

• derive the countdown from the server expiry on every tick and stop its timer at zero
• drive QR dimming and expired messaging from the same countdown state
• catch QR rendering failures and retain the canvas for later pairing URIs
• track concurrent device removals independently so only affected rows are disabled
• replace the hardcoded dashboard URL with a typed TanStack Router link
• add regressions for live expiry, QR rejection and recovery, concurrent removal state, and the config link
• merge the latest origin/main

Verification

• pnpm exec vitest run src/pages/MobilePairingPage.test.tsx (14 tests passed)
• pnpm test (103 files, 1094 tests passed)
• pnpm typecheck (passed)
• pnpm lint (passed)
• pnpm build (passed)
• pnpm test:i18n-parity reproduced only the existing main-branch baseline: eight extra plural keys in each of pl.json and uk.json; ko.json and zh.json passed
• git diff --check origin/main...HEAD (passed)

Out of scope

• pairing token lifetime and refresh API behavior remain server-owned
• device-removal transport semantics remain unchanged
• existing Polish and Ukrainian plural-key drift remains for a separate change