ihasmail 2.0: rebuild as Stalwart-first JMAP webmail

Replace the FastAPI/HTMX prototype with a Node/Hono session proxy and a
React 19/Vite SPA. Mail (conversation view, search operators, labels,
sanitised HTML, privacy image proxy, invites, undo send, templates),
calendar (month/week/day/agenda, invites, free/busy, categories,
context menus), contacts (JSContact, groups, vCard), files, Sieve filter
builder (incl. filter-from-message with retroactive apply), vacation,
identities with default + Reply-To, PWA/mobile layout, push via SSE,
in-memory mock Stalwart for dev, Docker + CI.

web/public/manifest.webmanifest

@@ -0,0 +1,21 @@
+{
+  "name": "ihasmail",
+  "short_name": "ihasmail",
+  "description": "Fast, friendly JMAP webmail for Stalwart",
+  "start_url": "/mail",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#ffffff",
+  "theme_color": "#0f766e",
+  "icons": [
+    { "src": "/img/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/img/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "/img/icon-maskable.png", "sizes": "192x192", "type": "image/png", "purpose": "maskable" }
+  ],
+  "shortcuts": [
+    { "name": "Compose", "url": "/mail?compose=new", "description": "Write a new message" },
+    { "name": "Calendar", "url": "/calendar" },
+    { "name": "Contacts", "url": "/contacts" }
+  ]
+}