Per operator request: instead of a disk-specific alert, surface a generic
server-warnings banner at the very top of every page, so new system
warnings can be added backend-side with no frontend change.
- hive-c0re `host_stats`: `server_warnings() -> Vec<ServerWarning>`
(`{ kind, level, message }`). The threshold logic lives server-side; the
host disk-pressure check (a `statvfs` probe of `/nix`: ≥85% used → warn,
≥95% → crit) is the first and only producer today. No new deps (libc).
- `/api/state` carries `server_warnings` (replaces the disk-specific
field). Empty when all clear.
- frontend: `renderServerWarnings` / `initServerWarnings` in `common.js`
inject a sticky top-of-<body> banner and render the list, coloured by
`level`. Wired on every page — dashboard (live, via refreshState),
FL0W, L0GS, H0M3. No per-warning frontend code; adding a warning kind
is a pure backend change.
cargo check/clippy/fmt + npm run build green. Closes#1518.
