refactor(daemon): use mtime-based version detection instead of string comparison

Replace CLI version string comparison with file modification time (mtime) based detection. This provides a more reliable way to detect when the CLI binary has been updated, especially for bun-compiled executables where package.json may not be accessible.

Changes:
- Add getInstalledCliMtimeMs() utility to check CLI binary or package.json mtime
- Store startedWithCliMtimeMs in daemon state on startup
- Use mtime comparison in version check loop for daemon auto-restart
- Move version flag handling to early CLI startup before daemon initialization
- Fix machine active state merging to prefer newer activeAt timestamp

This improves daemon restart behavior when CLI is updated via package managers.

server/src/sync/syncEngine.ts

@@ -476,13 +476,17 @@ export class SyncEngine {
             return { host, platform, happyCliVersion, displayName, ...data }
         })()
 
+        const storedActiveAt = stored.activeAt ?? stored.createdAt
+        const existingActiveAt = existing?.activeAt ?? 0
+        const useStoredActivity = storedActiveAt > existingActiveAt
+
         const machine: Machine = {
             id: stored.id,
             seq: stored.seq,
             createdAt: stored.createdAt,
             updatedAt: stored.updatedAt,
-            active: existing?.active ?? stored.active,
-            activeAt: existing?.activeAt ?? (stored.activeAt ?? stored.createdAt),
+            active: useStoredActivity ? stored.active : (existing?.active ?? stored.active),
+            activeAt: useStoredActivity ? storedActiveAt : (existingActiveAt || storedActiveAt),
             metadata,
             metadataVersion: stored.metadataVersion,
             daemonState: stored.daemonState,