perf(webapp): cache deployment logs across navigations (#4775)

Switching between deployments in the dashboard re-fetched the whole
build log stream from record zero and re-rendered the list line by line
every time. Logs are now cached per deployment for the lifetime of the
tab: revisiting a deployment shows its logs immediately, and the stream
is resumed from the next unread record rather than restarted. Finished
deployments whose stream has been read through the `finalized` event are
served entirely from the cache.

### Changes

The stream/cache logic moved out of the route into a `useDeploymentLogs`
hook. On each deployment switch it seeds state from the cache, resumes
the S2 read session at `nextSeqNum`, and writes back on cleanup or
natural session end. Completion is derived from the stream's own
`finalized` event (plus a terminal deployment status), not from the
session closing, so a session cut short by token expiry or a proxy
cannot pin a truncated log in the cache.

Memory is bounded by a small LRU (`deploymentLogsCache`): at most 20
deployments and 20,000 log lines in total, least recently viewed evicted
first. The most recently viewed deployment is always kept, so a single
very large log can temporarily exceed the line budget on its own.
Records are batched into one state update per tick instead of one per
line.