Precompress the bundle, validate the shell, and pass byte ranges on

The web build now writes a Brotli and a gzip copy of each compressible
file, and the static handler serves the best one the browser accepts.
The bundle was gzipped again for every request and Brotli was never
offered; the main chunk is 122 KB with Brotli against 144 KB gzipped.

index.html and every static file carry an ETag, and a matching
If-None-Match gets a 304. The shell and the worker are revalidated on
every load and were downloaded whole each time.

Attachment downloads pass a plain byte Range to Stalwart and relay a 206,
so a PDF viewer or a video element can read in pieces where the server
allows it. On the reader's own device a blob is cached as immutable,
since its id names its content.

The upstream session and account-info caches drop entries past their age
on a timer; they lost an entry only on sign-out or refusal, not when a
session expired. The mock answers byte ranges.

web/package.json

@@ -6,7 +6,7 @@
   "type": "module",
   "scripts": {
     "dev": "vite",
-    "build": "tsc -p tsconfig.json --noEmit && vite build",
+    "build": "tsc -p tsconfig.json --noEmit && vite build && node ../scripts/precompress.mjs dist",
     "preview": "vite preview",
     "typecheck": "tsc -p tsconfig.json --noEmit",
     "test": "vitest run"