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/announce.go

@@ -133,7 +133,7 @@ func (s *Server) handleAnnounce(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 
 	keyBytes, err := base64.StdEncoding.DecodeString(req.PublicKey)
 	if err != nil {
-		// Try URL-safe / no-padding flavors before giving up.
+		// Try URL-safe / no-padding flavours before giving up.
 		if k2, e2 := base64.RawStdEncoding.DecodeString(req.PublicKey); e2 == nil {
 			keyBytes = k2
 		} else {
@@ -195,7 +195,7 @@ func (s *Server) handleAnnounce(w stdhttp.ResponseWriter, r *stdhttp.Request) {
 // remoteIP returns r.RemoteAddr stripped of any :port suffix, plus
 // the X-Forwarded-For chain's first hop when behind a trusted proxy
 // (RM_TRUSTED_PROXY in the deployment doc). Trust-proxy lookup
-// matches the framework's existing behavior elsewhere.
+// matches the framework's existing behaviour elsewhere.
 func remoteIP(r *stdhttp.Request) string {
 	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
 		// Take the first IP in the chain (closest to the original