Serve ihasmail from a subpath

`BASE_PATH=/mail` mounts the whole app under a prefix, for a host that is not
ihasmail's alone. Unset -- every deployment that exists -- is the domain root
and is byte-for-byte what it was: the canonical form of the setting is the
empty string, and `""` concatenated onto `/api/health` is `/api/health`.

That choice of canonical form is the whole design. A trailing slash would have
been the obvious alternative, and it fails quietly in exactly one place: at the
root it makes `//api/health`, which is not a path on this host but a
protocol-relative URL to a host called `api`. One call site forgetting to
branch is a request leaving the origin. So the empty string, one leading slash,
no trailing one, worked out once in `scripts/basePath.mjs` -- plain JS, next to
`version.mjs`, because the web build and the server both have to reach the same
answer and two implementations of "what does /mail/ mean" is precisely the bug
where the server serves an app whose script tags point somewhere else.
`/mail`, `mail`, `/mail/` and `//mail//` all mean the same mount; a deployment
should not fail over a trailing slash.

Unlike everything else ihasmail is told, this one cannot wait for the process
to start. The bundle writes its own asset URLs into index.html, so `BASE_PATH`
is read at build time for Vite's `base` as well as at run time for the routes,
and the Dockerfile carries one value into both. Get them out of step and the
page comes up blank with a 404 in a console nobody has open -- so the static
handler, which is reading index.html anyway, checks what it asks for and says
so in the log once per build.

Everything moves together. The API mounts at `${base}/api`; the router is
given the base once, so every `<Route path>` and `<Link href>` stays written
root-absolute and wouter does the rest; `apiFetch` adds the prefix in one place
rather than at forty call sites; the session cookie's Path narrows to the mount
so two instances on one host cannot sign each other out.

Two things need no prefix at all, and it is worth saying why they were not
given one. A manifest's members resolve against the manifest's own address, so
relative URLs there follow the mount with nothing substituted at build time --
which is also why `public/` needed no template step. The service worker is the
same trick: it is served from the mount, so `new URL("./", self.location)`
tells it where that is, and a worker that derives the value cannot disagree
with the page that registered it.

Anything outside the mount is a 404 rather than the app shell, and
`stripBasePath` does not use `startsWith` -- under `/mail` this process shares
a hostname, and answering `/mailbox` with our index would shadow a neighbour
instead of letting it 404 honestly. For the same reason the notification-click
handler now checks the path as well as the origin: `includeUncontrolled` widens
`matchAll` to the whole origin, which off the root would have navigated a
stranger's tab to our inbox.

Inline images in a draft were the one silent trap. They are matched by their
blob URL on the way out, once unanchored and once anchored, and a bare
`/api/blob/` still appears inside `/mail/api/blob/...` -- so one pattern would
have replaced the tail and left `/mail` in front of a `cid:`, and the other
would have missed and sent the message linking to the sender's own webmail.
Both patterns are built from the base now.

web/src/lib/basePath.ts

@@ -0,0 +1,29 @@
+/**
+ * The subpath this build is mounted at, as the browser sees it.
+ *
+ * `import.meta.env.BASE_URL` is Vite's own copy of the `base` it built with,
+ * and `vite.config.ts` sets that from `BASE_PATH` through the shared
+ * normaliser -- so this is the same answer the server reached, not a second
+ * guess at it. Reading it here rather than re-deriving it from
+ * `window.location` matters because the app is a SPA: at `/mail/inbox/abc`
+ * there is nothing in the address that says how much of it is the mount.
+ *
+ * Vite guarantees the value ends in a slash, so the only conversion is
+ * dropping it; `""` for the root, `/mail` otherwise, matching
+ * `scripts/basePath.mjs`.
+ */
+export const BASE_PATH: string = import.meta.env.BASE_URL.replace(/\/+$/, "");
+
+/**
+ * Turn a root-absolute app path into one the server will answer.
+ *
+ * Every `/api/...`, `/img/...` and `/sw.js` in the app goes through here.
+ * Router paths do not: wouter is given `BASE_PATH` as its base and strips and
+ * re-adds the prefix itself, so `<Link href="/mail">` stays written that way.
+ * Mixing the two would double the prefix, which is why this asserts nothing
+ * and simply concatenates -- the discipline is at the call sites, and the
+ * callers that need it are few and all in this repo.
+ */
+export function withBase(path: string): string {
+  return BASE_PATH + path;
+}