feat(fe): keep an app's session across a page load

[sea-snake]

Design: #4224. Overview: #4230. Consumed by #4273, which stores a record, and #4248, which reads and discards one.

A session outlives the tab that created it. Without storing the chain, a sibling subdomain asking for the first time — or a user returning to a closed tab — would mean another ceremony, which is the cadence this design exists to remove.

Records keyed by identity, account and origin, in IndexedDB alongside the identity's own session.

A record within five minutes of its expiry is not treated as usable, so a chain is never handed over that dies mid-request. Five minutes is half the shortest session a caller can ask for, so a session near that floor is never answered from storage.

The purge that already runs when an identity is discarded now takes these records with it, at the eight places that discard one. Leaving them would keep a chain for an identity the browser has forgotten.

Three of the four exports have no caller here: storeAppSession gains one in #4273, appSessionsForOrigin and discardAppSession in #4248. purgeAppSessions is called by this PR.

Tests: app-session.store.test.ts (7): storage keyed by identity, account and origin; a record for another origin not served; an expiring record not served; and the purge removing them.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 9b420ec.

Security Overview

• 🔎 Scanned files: 7 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/6d3e3b61-710b-4b2b-9c0b-43458b6314b6?scanId=73cea0f2-1e64-4d7c-855d-dbda1f1ba11e&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► src/frontend/src/lib/stores/app-session.store.ts     Add in-app session store and management (storeAppSession, discardAppSession, appSessionsForOrigin, purgeAppSessions)
Enhancement	► src/frontend/src/routes/(new-styling)/+page.svelte     Invoke purgeAppSessions on identity removal
Enhancement	► src/frontend/src/routes/(new-styling)/authorize/views/ContinueView.svelte     Invoke purgeAppSessions on certain error paths
Enhancement	► src/frontend/src/routes/(new-styling)/cli/+layout.svelte     Invoke purgeAppSessions on identity removal
Enhancement	► src/frontend/src/routes/(new-styling)/manage/(authenticated)/+layout.svelte     Invoke purgeAppSessions on identity removal
Enhancement	► src/frontend/src/routes/(new-styling)/recovery/+page.svelte     Invoke purgeAppSessions on identity removal