Fix "Add variable" doing nothing, and size the editors to the viewport (0.42.2)

The env table derived its rows from the serialized text on every render,
and serialize() drops rows with an empty key — so a freshly added blank
row was discarded before it could be typed into. The rows are now owned
by the component and re-parsed only when `value` changes from outside,
with stable per-row ids so deleting a row no longer shifts the reveal
state onto its neighbour.

AppShell's <main> is content-height, so the editor page's `h-full`
collapsed to auto: Monaco and the raw .env textarea fell back to their
intrinsic size, the textarea to a two-row default. The page is now sized
against the viewport minus the top bar and page padding, so both editors
fill the screen, and the textarea gets min-h-0 so flex-1 can grow it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016pMmFFkdfxkoYjcEcpZTa5

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.42.1",
+  "version": "0.42.2",
   "type": "module",
   "scripts": {
     "dev": "vite",