### Problem
Webapp HTML references content-hashed /build assets, and each Docker
image contains exactly one build with a hard 404 for unknown hashes.
During a rolling deploy, a client holding HTML from the old build may
request old asset hashes from a replica running the new image, causing
missing styles or failed chunk loads.
The page should recover automatically once a compatible build becomes
available, without reload loops or unnecessary interruptions during
normal deployments.
### What changed
- Build changes alone do nothing — no polling, no automatic reloads.
- If a CSS or JavaScript asset fails to load, a recovery overlay is
shown immediately.
- While the server still reports the same build, the client polls for a
newer build using exponential backoff (up to ~60s). As soon as a newer
build is detected, the page reloads automatically.
- If no newer build appears within the timeout, recovery falls back to a
manual Reload action.
- If recovery still fails after the automatic reload, the client stops
retrying and displays a final recovery screen instead of entering a
reload loop.
- Recovery preserves form values and scroll position across the
automatic reload.
Katia Bulatova
2aa64200f8