fix(pwa): bump field-shell cache to v2 — evicts stale UI for installed PWAs

The Apple HIG polish pass (liquid-glass chrome, iOS SegmentedControl,
ThresholdMeter) was already on origin/main and live, but anyone with
the /water PWA installed on their home screen kept seeing the OLD UI
because the SW cache name never bumped.

Old SW had field-shell-v1 baked into its bytes for every prior deploy.
The activate handler only deletes caches that aren't SHELL_CACHE or
DATA_CACHE and start with 'field-'. Same name = no eviction = stale
HTML + chunks forever.

Bumping SHELL_CACHE to 'field-shell-v2' makes the SW bytes change,
which triggers the normal SW upgrade flow:
  install → skipWaiting → activate → claim clients
The activate handler then evicts field-shell-v1 (it's now a foreign
field-* cache). On next navigation the new SHELL_CACHE is repopulated
with the fresh /water HTML + static assets, and the user gets the new
UI.

public/sw-field.js

@@ -12,7 +12,12 @@
 // Cache names are deliberately prefixed with `field-` so they never
 // collide with the main `rc-shell-v*` / `rc-data-v*` caches.
 
-const SHELL_CACHE = "field-shell-v1";
+// Bumped to v2 after the Apple HIG polish pass (liquid-glass chrome,
+// iOS SegmentedControl, ThresholdMeter). The activate handler keeps
+// only this name + DATA_CACHE — anything else starting with `field-`
+// (including the old v1) is evicted on the next SW activation, which
+// happens automatically because the bytes of this file changed.
+const SHELL_CACHE = "field-shell-v2";
 const DATA_CACHE = "field-data-v1";
 const OFFLINE_URL = "/offline.html";