Keep cached pages usable on slow or failing networks

[brsbl]

Human comments

• the goal is to allow users to keep using bb in a read-only way when they have slow/spotty network. today, bb can get pretty unusable in these scenarios (eg driving in a short tunnel)
• this PR caches loaded pages while fetching new data in the background, and only invalidates the cache if the request succeeds.

What was wrong

Blocking config reads and error-first rendering hid cached data on slow or failing networks. Thread caches expired after five minutes, and failed page-code downloads escaped to the app-wide error screen.

What changed

• Cached config, sidebar, project, plugin, skill and automation content remains visible through background refreshes; successful responses update it.
• Returning threads display cached messages for up to 30 minutes of inactivity while newer data loads.
• Failed page-code downloads show a page error with Reload page, keeping app navigation usable.

Uses existing query caches, subscriptions and error boundaries. Mutations and public APIs are unchanged. Thread caching is in memory; hard reloads need the network. Page-download recovery requires the app shell to have loaded.

How you verified

• Remote CI passed at 94d252b6b: cached reads, refresh success/failure, cache misses, retention and page-error recovery.
• Chrome for Testing 153.0.8010.52, 390 × 844 @2×: delayed reads, injected 503s and recovery passed across changed surfaces.
• Fresh five-panel Slop Cop found no P0/P1 issues; two P2 follow-ups are recorded below.

States screenshots cannot establish alone

State over time	Before	After	Verification
Stale config: pending → success or failure	Blocked on the read.	Return cached config immediately; apply successful updates through existing subscriptions.	Remote pending/success/failure regressions passed.
Persisted sidebar: failed refresh → retry	Placeholder content could disappear.	Keep the snapshot until a successful response replaces it.	Remote replay, failure/retry and invalidation regressions passed; browser reload/recovery passed.
Thread return after five minutes	Cache eviction caused a loading skeleton.	Render cached messages during refresh; keep them after failure.	After 5m 9s away: 34 rows visible in 176 ms with ten reads held; all survived 503, then refreshed successfully.
Cached project/plugin/skill/automation: failure → success	Failed reads hid or cleared content.	Keep the last successful content until refresh succeeds.	Remote regressions and browser 1.2–1.6s delays/503/recovery passed; skill and automation updates appeared.
Page-code failure: navigate away → return → reload	App-wide error removed navigation and persisted after connectivity returned.	Keep navigation usable; reload recovers the failed page.	Actual Settings module 503: Back to app opened the composer; return stayed contained; reload restored General settings.

Before / after screenshots

Merge base 7bbff00 → PR head 94d252b; matching fixtures, routes, interaction states and 390 × 844 @2×. Thread screenshots show requests held after five minutes away; the page-code pair shows a Settings module 503; other pairs show failed background reads. Registry content uses a deterministic fixture. Fresh captures use Chrome for Testing 153.0.8010.52; retained base captures for thread, plugin settings, defaults and both skill views use 153.0.8010.36.

Surface	Before	After
Failed page-code download
Thread return with requests held
Projects
Project details
Installed plugins
Plugin details
Plugin settings
Project thread defaults
Registry skill source
Local skill file
Automation detail

P2 follow-ups

The fresh review covered all 23 changed files at 94d252b6b against 7bbff004b. These P2s are deferred under the requested review policy:

• Partial sidebar replay: after bootstrap failure, the 30-thread persisted snapshot can suppress child/mention fallback reads. Distinguish partial replay from complete bootstrap data.
• Sidebar freshness: realtime patches can mark an unrefreshed snapshot indefinitely fresh. Preserve staleness until a complete bootstrap succeeds.

BB-Thread-ID: thr_3mafhqizp2

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