fix(deploy): kiosk picks up new builds without operator intervention

Reproduced live during the #1133 rollout: the SpoolBuddy display kept
  serving the pre-fix picker for hours after every cache-clear,
  chromium-restart, and pkill attempt because a chain of stale state
  across HTTP cache + Service Worker + persistent profile prevented
  fresh code from reaching the running tab.

  Three independent changes — any one of them sufficient on a clean
  profile, but all three needed to escape an already-corrupted one:

  (1) backend/app/main.py — index.html now served with
  Cache-Control: no-cache, must-revalidate on both / and the SPA
  catch-all. Vite emits content-hashed JS/CSS bundle filenames so the
  assets themselves are safe to cache forever, but the HTML wrapping
  them is the only file that knows which hash is current. Without
  explicit cache directives Chromium falls back to heuristic caching
  (typically 10% of time since Last-Modified) and on long-running
  kiosks happily serves stale HTML across browser restarts. That stale
  HTML references an old bundle hash which is also still in disk
  cache, so the kiosk runs pre-deploy JS forever without ever knowing
  why.

  (2) frontend/public/sw.js — CACHE_NAME bumped from bambuddy-v25 to
  bambuddy-v26 so any client that fetches the new sw.js drops its old
  CacheStorage. The SW does network-first for HTML/JS/CSS but
  intercepts and falls back to cache, and cache-control on HTTP
  responses doesn't reach into the SW's own cache layer.

  (3) spoolbuddy/install/install.sh — generated kiosk launcher now uses
  --user-data-dir=/tmp/spoolbuddy-kiosk-userdata with a pre-launch
  rm -rf, so every kiosk restart starts from a clean slate (no HTTP
  cache, no SW registration, no IndexedDB). Trade-off is a slightly
  slower first paint and zero offline support; neither matters for a
  single-purpose kiosk facing a backend on the same LAN, and the
  guarantee that next-deploy-just-works is worth far more.

  4 new tests in test_static_html_cache_headers.py: index.html on /
  and SPA catch-all paths emit Cache-Control: no-cache,
  must-revalidate; API routes are unaffected (no leak of HTML cache
  directive onto endpoints we want React Query to cache aggressively).

  For existing kiosks already trapped by an old persistent profile,
  operator runs once: rm -rf ~/.config/chromium && systemctl restart
  getty@tty1.service. The new launcher then picks up automatically.

static/sw.js

@@ -1,5 +1,5 @@
 // Bambuddy Service Worker
-const CACHE_NAME = 'bambuddy-v25';
+const CACHE_NAME = 'bambuddy-v26';
 const STATIC_CACHE = 'bambuddy-static-v25';
 
 // Static assets to cache on install