chore(panel): fleet-aware shell footer + drop dead vuefinder dep

COA-B cleanup:
- Sidebar agent-health footer now reads the fleet store (host count / online
  count / per-host status + last heartbeat) instead of the single legacy
  server.connection row, which disagreed with the multi-host fleet. Removed the
  legacy useServerStore dependency from the shell.
- Removed the unused 'vuefinder' dependency (replaced by the native file
  manager): dep + main.ts plugin/CSS registration. Main JS chunk 588kB -> 165kB.

Recon reclassified the 'dead cmd.server v1' item: it is the LIVE license-level
command path (module config applies, plugin install, schedules, legacy
start/stop) served only by the Go agent — a Rust-agent parity gap, not dead
code. Left intact.

Build-green (vue-tsc) + boots clean in-browser (0 console errors).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -14,8 +14,7 @@
     "pinia": "^3.0.4",
     "pinia-plugin-persistedstate": "^4.7.1",
     "vue": "^3.5.25",
-    "vue-router": "^5.0.2",
-    "vuefinder": "^4.1.1"
+    "vue-router": "^5.0.2"
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.1.18",