0.35.0: per-stack Update button on the Stacks page

Adds an inline "Update (pull latest images & recreate)" action to each
row of the stacks table, next to start/stop/restart/edit — for both the
local host and remote agents. Wires the existing updateImages action and
the agent "update" lifecycle action through StacksTable's new onUpdate prop.

Also bumps backend/version.py to 0.35.0 so it tracks the frontend version
again (it had drifted to 0.33.0 while package.json moved to 0.34.x).

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.34.1",
+  "version": "0.35.0",
   "type": "module",
   "scripts": {
     "dev": "vite",