P3 follow-up: editable target dir, conditional --no-ownership, UK lint

Three small follow-ups from review:

1. Restore target is now operator-editable. Default value is the
   literal '\$HOME/rm-restore/<job-id>/' (agent expands \$HOME at
   run time using os.UserHomeDir(); also handles \${HOME} and ~/
   prefixes). Operator can replace with any absolute path.
   - ui_restore.go validates the input is either absolute or starts
     with one of the recognised prefixes; other env-var refs (\$PATH
     etc.) are deliberately rejected so operator paths can't pick up
     arbitrary agent env values.
   - host_restore.html replaces the read-only mono-text display with
     a real <input>; help text spells out that \$HOME resolves
     agent-side and <job-id> is substituted on dispatch.
   - install.sh + the systemd unit prep /root/rm-restore so the
     default works under the sandbox: ReadWritePaths gains a soft
     '-/root/rm-restore' entry (the '-' makes the bind-mount soft-fail
     if missing, but install.sh pre-creates it root-owned 0700).

2. --no-ownership flag now gated on restic version. The flag was
   added in restic 0.17 and 0.16 rejects it. Previously dropped it
   wholesale — that meant new-dir restores silently preserved
   ownership against design intent on 0.17+. Now the agent threads
   its detected restic version (sysinfo already collects it) through
   runner.Config -> restic.Env, and RunRestore appends --no-ownership
   only when AtLeastVersion(0, 17) returns true. 0.16 hosts still
   restore with original uid/gid; help text in the wizard explicitly
   notes this. The previous 'Original ownership is preserved' copy
   was wrong for new-dir mode and is corrected.

3. golangci-lint misspell locale switched US -> UK and the codebase
   swept (73 corrections, mostly behaviour/serialise/recognise/honour).
   Wire-format ErrorCode 'unauthorized' -> 'unauthorised' is a tiny
   contract change but the agent doesn't parse those codes today and
   no external API consumers exist yet. Tests passed before + after.

Tests:
- internal/restic/version_test.go covers Env.AtLeastVersion across
  edge cases (empty, exact match, patch above, minor below, non-
  numeric) and expandHome on \$HOME / \${HOME} / ~/, plus
  pass-through for absolute paths and refusal of other env vars.
- ui_restore_test updated: TargetDir now starts '\$HOME/rm-restore/'
  with the job_id substituted into the placeholder.

Live verified on the smoke env: default target restored to
/root/rm-restore/<job-id>/ as the agent's expanded \$HOME (2 files,
14 bytes); custom override '/tmp/custom-restore/<job-id>/' restored
into the agent's PrivateTmp namespace (1 file, 6 bytes); both jobs
'succeeded', exit 0.

internal/server/http/host_credentials.go

@@ -32,7 +32,7 @@ type hostRepoCredsView struct {
 // creds for UI display. 404 if no credential has ever been set.
 func (s *Server) handleGetHostCredentials(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	if !s.authedUser(r) {
-		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorized", "")
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
 		return
 	}
 	hostID := chi.URLParam(r, "id")
@@ -88,7 +88,7 @@ type hostRepoCredsRequest struct {
 func (s *Server) handleSetHostCredentials(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	user, ok := s.requireUser(r)
 	if !ok {
-		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorized", "")
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
 		return
 	}
 	hostID := chi.URLParam(r, "id")
@@ -165,7 +165,7 @@ func (s *Server) handleSetHostCredentials(w stdhttp.ResponseWriter, r *stdhttp.R
 	w.WriteHeader(stdhttp.StatusNoContent)
 }
 
-// pushRepoCredsToAgent serializes blob into a config.update envelope
+// pushRepoCredsToAgent serialises blob into a config.update envelope
 // and ships it down the agent's WS. Returns an error from the hub
 // (no-op if not connected — caller is expected to check first when it
 // matters).
@@ -192,7 +192,7 @@ func (s *Server) pushRepoCredsToAgent(ctx context.Context, hostID string, blob r
 // uses this to pre-fill the edit form.
 func (s *Server) handleGetAdminCredentials(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	if !s.authedUser(r) {
-		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorized", "")
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
 		return
 	}
 	hostID := chi.URLParam(r, "id")
@@ -234,7 +234,7 @@ func (s *Server) handleGetAdminCredentials(w stdhttp.ResponseWriter, r *stdhttp.
 func (s *Server) handleSetAdminCredentials(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	user, ok := s.requireUser(r)
 	if !ok {
-		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorized", "")
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
 		return
 	}
 	hostID := chi.URLParam(r, "id")
@@ -319,7 +319,7 @@ func (s *Server) handleSetAdminCredentials(w stdhttp.ResponseWriter, r *stdhttp.
 func (s *Server) handleDeleteAdminCredentials(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 	user, ok := s.requireUser(r)
 	if !ok {
-		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorized", "")
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
 		return
 	}
 	hostID := chi.URLParam(r, "id")