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/agent/runner/runner.go

@@ -26,10 +26,11 @@ type Sender interface {
 // from the agent's config file (server-pushed config.update payloads
 // override these in memory).
 type Config struct {
-	ResticBin    string
-	RepoURL      string
-	RepoUsername string
-	RepoPassword string
+	ResticBin     string
+	ResticVersion string // e.g. "0.17.1" — empty if unknown
+	RepoURL       string
+	RepoUsername  string
+	RepoPassword  string
 
 	// Bandwidth caps in KB/s applied to every restic invocation.
 	// <=0 means "no cap". Per-job override: callers that build a
@@ -61,6 +62,7 @@ func New(cfg Config, tx Sender, progressMinPeriod time.Duration) *Runner {
 func (r *Runner) resticEnv() restic.Env {
 	return restic.Env{
 		Bin:               r.cfg.ResticBin,
+		Version:           r.cfg.ResticVersion,
 		RepoURL:           r.cfg.RepoURL,
 		RepoUsername:      r.cfg.RepoUsername,
 		RepoPassword:      r.cfg.RepoPassword,