fix: P0+P1 critical bug fixes across controller (24 files)

Concurrency fixes:
- Deep-copy stacks in GetStack/GetStacks to prevent shared state mutation (C04)
- Add per-state mutex to watchdog pathProbeState (C05)
- Guard MetricsCollector.Start() with sync.Once against double-start (C06)
- Hold diskJobMu across entire raw mount operation (C07)
- Add mutex to SetEncryptionKey (C08), MigrateEncryption write lock (H03)
- Use sync.Once for sync.Stop() channel close (H08)
- Set syncing=true before releasing lock in TriggerSync (H09)
- Deep-copy lastDBDump/lastBackup in GetFullStatus (H11)
- Add WaitGroup for stderr goroutine in MigrateDrive (H19)
- Add mutex to SetBackupRunningCheck (M18)

Security fixes:
- Validate Bearer token against Hub API key in CSRF middleware (H16)
- Validate backup paths start with expected prefix in RemoveStack (M12)
- Guard uuid[:8] slice with length check (H20)
- Parse fstab fields exactly for mount target matching (H21)

Bug fixes:
- Use decrypted env vars for compose deploy (C01)
- Log decrypt failures in DecryptMap instead of swallowing (C02)
- Move Deployed=false inside lock in runComposeDeploy (C03)
- Fix activeDrives() to skip disconnected drives (H02)
- Fix Snapshot() stderr extraction from exec.ExitError (H01)
- Check unlockCmd.Run() error in restic (H01)
- Buffer template rendering via bytes.Buffer (H07)
- Thread context.Context through cloudflare client (H10)
- Fix leaf-name collision detection in cross-drive backup (H15)
- Add nil check for crossDriveRunner (H17)
- Use strings.TrimSpace instead of slice on command output (H18)
- Make SaveAppConfig atomic with write-to-tmp+rename (H04)
- Pass encKey on deploy failure SaveAppConfig (H05)
- Fix IPv6 address format in TCP health probe

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/web/server.go

@@ -1,6 +1,7 @@
 package web
 
 import (
+	"bytes"
 	"fmt"
 	"html/template"
 	"log"
@@ -391,11 +392,14 @@ func (s *Server) primaryHDDPath() string {
 }
 
 func (s *Server) render(w http.ResponseWriter, name string, data interface{}) {
-	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	if err := s.tmpl.ExecuteTemplate(w, name, data); err != nil {
+	var buf bytes.Buffer
+	if err := s.tmpl.ExecuteTemplate(&buf, name, data); err != nil {
 		s.logger.Printf("[ERROR] Template error (%s): %v", name, err)
 		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
 	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	buf.WriteTo(w)
 }
 
 // executeTemplate renders a template with CSRF data auto-injected into the data map.
@@ -406,11 +410,14 @@ func (s *Server) executeTemplate(w http.ResponseWriter, r *http.Request, name st
 	}
 	data["CSRFField"] = s.csrfField(r)
 	data["CSRFToken"] = s.csrfToken(r)
-	w.Header().Set("Content-Type", "text/html; charset=utf-8")
-	if err := s.tmpl.ExecuteTemplate(w, name, data); err != nil {
+	var buf bytes.Buffer
+	if err := s.tmpl.ExecuteTemplate(&buf, name, data); err != nil {
 		s.logger.Printf("[ERROR] Template error (%s): %v", name, err)
 		http.Error(w, "Internal error", http.StatusInternalServerError)
+		return
 	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	buf.WriteTo(w)
 }
 
 // --- Static file / asset serving ---