Give builds a version number

ihasmail called itself "2.0" on the About page and "2.0.0" from
/api/health, both hardcoded, in four places that had drifted from each
other and from anything meaningful. A build now says what it is:

  ihasmail v2.16.57
             |  |  |
             |  |  the pull request the commit came from
             |  the Stalwart generation this build targets -- 0.16
             ihasmail's own major

The first two are the version in the root package.json, so there is a
single place to bump them, and 16 becomes 17 when ihasmail moves to
Stalwart 0.17. Dropping 0.15 is what makes that middle number honest:
while two generations were supported it could not have been either.

The pull request number comes from git at build time and is never
written back into the tree. It cannot be: it does not exist until the
pull request has merged, so a committed version would always describe a
merge that had not happened yet, and every open branch would collide on
the same line. A commit that did not come through a pull request carries
the last number plus its own short SHA -- 2.16.57+g1fa6578 -- which says
it is past that pull request rather than quietly claiming to be it.

.dockerignore excludes .git on purpose, so an image build cannot work
any of this out. It takes --build-arg IHASMAIL_VERSION instead, which
the build stage bakes into the bundle and the runtime stage keeps as an
environment variable for the server. Left out, it falls back to the base
version from package.json rather than failing -- so a version with no PR
number on it means whoever built the image did not pass one.

scripts/ is copied into the runtime image because the server resolves
its version through it. There is no git in there to ask, which is the
fallback's whole purpose.

Verified: 2.16.57 in the bundle and from /api/health on a dev checkout;
the same after a real docker build --build-arg, from inside the
container; and 2.16.0 rather than a crash when the arg is left off.

Note for deploying: ihasmail-deploy.sh on the host builds without the
argument and will produce 2.16.0 until it passes
--build-arg IHASMAIL_VERSION="$(node scripts/version.mjs)".

README.md

@@ -151,6 +151,40 @@ npm start              # serve the production build
 
 Open http://localhost:5173 in dev (or http://localhost:8080 for the production build).
 
+### Version numbers
+
+`ihasmail v2.16.57`, as shown in Settings › About and by `/api/health`:
+
+| | |
+| --- | --- |
+| `2` | ihasmail's own major |
+| `16` | the **Stalwart** generation this build targets — 0.16, the oldest it supports |
+| `57` | the pull request the commit came from |
+
+The first two live in the root `package.json`, so there is one place to bump
+them; `16` becomes `17` when ihasmail moves to Stalwart 0.17. The third comes
+from git at build time, because it does not exist until the pull request has
+merged — a version committed to the tree would always be describing a merge
+that had not happened yet, and every open branch would collide on the same
+line. Nothing writes one back.
+
+A commit that did not arrive through a pull request carries the last number
+plus its own short SHA — `2.16.57+g1fa6578` — which says plainly that the build
+is *past* that pull request rather than being it.
+
+`node scripts/version.mjs` prints the version for the current checkout.
+
+`.dockerignore` excludes `.git` deliberately, so an image build cannot work any
+of this out for itself. Pass it in:
+
+```bash
+docker build --build-arg IHASMAIL_VERSION="$(node scripts/version.mjs)" -t ihasmail:2.16 .
+```
+
+Left out, the build falls back to the base version from `package.json`
+(`2.16.0`) rather than failing — so a version with no PR number on it means
+whoever built the image did not pass one.
+
 ### The mock
 
 `npm run mock` is an in-memory fake Stalwart 0.16 — enough of JMAP to develop