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.

scripts/precompress.mjs

@@ -0,0 +1,47 @@
+#!/usr/bin/env node
+/*
+ * Write a Brotli and a gzip copy beside every compressible file in a web build.
+ *
+ * The server used to gzip the bundle again on every request that asked for it,
+ * at a level chosen for speed. These are made once, at the level chosen for
+ * size, and `server/src/static.ts` hands one out when the browser accepts it.
+ * Brotli at 11 is about 15% smaller than gzip for this bundle, and too slow to
+ * do per request, which is why it was never offered.
+ *
+ *   node scripts/precompress.mjs web/dist
+ */
+import { readdirSync, readFileSync, statSync, writeFileSync } from "node:fs";
+import { join, extname } from "node:path";
+import { brotliCompressSync, constants, gzipSync } from "node:zlib";
+
+const COMPRESSIBLE = new Set([".js", ".mjs", ".css", ".html", ".svg", ".json", ".webmanifest", ".txt", ".wasm"]);
+// Below this, the encoding costs more than it saves.
+const MIN_BYTES = 1024;
+
+function* files(dir) {
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const p = join(dir, entry.name);
+    if (entry.isDirectory()) yield* files(p);
+    else yield p;
+  }
+}
+
+const root = process.argv[2];
+if (!root) {
+  console.error("usage: precompress.mjs <dir>");
+  process.exit(2);
+}
+let count = 0;
+let before = 0;
+let after = 0;
+for (const p of files(root)) {
+  if (!COMPRESSIBLE.has(extname(p)) || statSync(p).size < MIN_BYTES) continue;
+  const data = readFileSync(p);
+  const br = brotliCompressSync(data, { params: { [constants.BROTLI_PARAM_QUALITY]: 11, [constants.BROTLI_PARAM_SIZE_HINT]: data.length } });
+  writeFileSync(`${p}.br`, br);
+  writeFileSync(`${p}.gz`, gzipSync(data, { level: 9 }));
+  count++;
+  before += data.length;
+  after += br.length;
+}
+console.log(`precompressed ${count} files: ${(before / 1024).toFixed(0)} KB -> ${(after / 1024).toFixed(0)} KB brotli`);