Fix git_info flash, deleted-build error, and browser TUI corruption

[bluestreak01]

Summary

Three small bugfixes surfaced while using the app today:

• Panel git info no longer flashes to None when the filesystem watcher fires. reload_panels was calling GitCache::invalidate, which removed the repo's cache entry — so the very next refresh_git returned None and the panel blanked out for a frame until the background query finished. Replaced invalidate with refresh_async, which spawns the background query but leaves the existing entry in place. poll_pending swaps in the fresh data when it lands.
• Azure DevOps timeline now reports deleted builds clearly. When a build has been purged by retention policy (or deleted manually), the timeline endpoint returns HTTP 404 with a BuildNotFoundException JSON body. The previous code only branched on 401/403 auth errors, so 404 fell through to serde_json::from_str::<Timeline> and surfaced Failed to parse Azure timeline: missing field \records`. Now we extract the server's message` field (e.g. "The requested build N has been deleted.") and return it directly.
• Browser launch no longer corrupts the TUI. open_url was using Command::spawn() without redirecting stdio, so Firefox's "Opening in existing browser session." stderr message (and any Gtk/Wayland warnings) bled onto the TUI when pressing o in the CI panel or during OAuth login. Now we redirect stdin/stdout/stderr of the spawned xdg-open/open process to /dev/null.

Test plan

• Enter a directory inside a git repo — git info appears immediately and stays (no blink)
• Open a CI check whose Azure DevOps build has been deleted — see "The requested build … has been deleted." instead of the parse error
• Press o on a CI check while a browser instance is already running — TUI stays clean, no "Opening in existing browser session." text on screen

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