v0.4.64: NFS mount diagnostics — pre-flight probe, retry, hint translation

The dominant field failure from v0.4.63 was "mount.nfs: failed to apply
fstab options" (exit 32), surfaced verbatim by the Storage tab. The
message is misleading — it has nothing to do with /etc/fstab; it comes
from nfs-utils 2.6.x's nfs_options2string() and most commonly indicates
the container is missing CAP_SYS_ADMIN, /etc/mtab is unwritable, or an
auxiliary option triggered an option-transform edge case.

Backend (crates/iso-store/src/nfs.rs):
- TCP pre-flight probe to server:port (4s timeout) before shelling out.
  Catches wrong-IP / firewall cases as "cannot reach NFS port" instead
  of letting mount.nfs spit out an unhelpful message.
- proto=tcp explicit on NFSv3 (UDP is widely deprecated, modern NAS
  appliances often don't bind UDP at all).
- Optional `port` field on NfsAddRequest (defaults to 2049), persisted
  on NfsMount.
- On "failed to apply fstab options" / "internal option parsing error"
  retry with a minimal option set (vers=N,ro/rw only) — bypasses the
  nfs-utils transformation bug; if it still fails we get a real kernel
  error to translate.
- hint_for() translates well-known stderr patterns into actionable
  guidance — CAP_SYS_ADMIN for option-transform failures, exports-table
  for access-denied, export-path hint for "no such file or directory"
  (calling out the UniFi UNAS Pro /var/nfs/shared/<name> convention),
  etc.
- normalize_server() strips http://, https://, nfs:// schemes the
  operator may have pasted by mistake, plus trailing slashes.

API (crates/http-api/src/app.rs):
- api_nfs_add now returns a structured {error, stderr, hint} JSON body
  on failure instead of plain text. UI renders the error in bold with
  the hint as a dimmer second line.

UI (crates/webui/src/app.js):
- Storage tab's "Mount failed" banner now shows the raw error + hint on
  two lines. Each persisted mount row also surfaces last_hint under
  last_error.

Terminal (crates/http-api/src/terminal.rs):
- `nfs mount` command prints "hint: ..." on a follow-up line when the
  manager returns one.

Tests:
- 8 new tests covering option string (incl. proto=tcp on v3, port=N for
  non-default), minimal-options stripping, server normalization, and
  hint translation for each well-known stderr pattern.
- All 150 tests pass; clippy -D warnings clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

crates/http-api/src/app.rs

@@ -1713,11 +1713,12 @@ async fn api_nfs_list(State(state): State<AppState>) -> Json<serde_json::Value>
 async fn api_nfs_add(State(state): State<AppState>, Json(req): Json<NfsAddRequest>) -> Response {
     match state.nfs.add(req).await {
         Ok(m) => (StatusCode::CREATED, Json(m)).into_response(),
-        // Anything from the manager surfaces as a user-fixable validation
-        // error — bad host, kernel without NFS support, missing
-        // `mount.nfs`, dead server. We pass the message through verbatim
-        // so the UI can show it to the operator.
-        Err(e) => (StatusCode::BAD_REQUEST, format!("{e}")).into_response(),
+        // v0.4.64: the manager returns a structured `NfsMountError` with
+        // `error` + optional `hint` + the raw `stderr`, so the UI can
+        // show both — the raw message for completeness, the hint for
+        // "what to fix next". Previously this was a plain text body
+        // which collapsed both bits of information into one line.
+        Err(err) => (StatusCode::BAD_REQUEST, Json(err)).into_response(),
     }
 }