Start the 0.4 line, and give previews the version they are previewing

Two version problems, one of them mine.

**Previews claimed x.y.0.** The preview workflow hard-coded the patch
number, so every preview installer reported 0.3.0 whatever it contained,
while the real build computes the patch from tags. It now runs the same
computation, so a preview is labelled with the version the release it
previews would carry.

**A new minor line started at the wrong number.** `compute-version`'s
fallback for "no tag matches this line yet" counted every commit in the
repository — fine as a bootstrap, wrong the moment a minor version is
bumped: the first 0.4 build would have been 0.4.234. A line nobody has
tagged is a new line, and a new line starts at .0.

With those fixed, VERSION moves to 0.4 — tab reordering, the browser
pop-out, opening pages in the container's browser and the Playwright
install fix are more than a patch bump. The next release is v0.4.0;
today's HEAD would have been 0.3.90 on the old line.

`app/package.json`, `package-lock.json`, `tauri.conf.json` and
`Cargo.toml` follow to 0.4.0. CI patches all four per build, so they are
the dev-time defaults rather than the source of truth — but a local
`tauri dev` shows them, so they should not still say 0.3.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-app.yml

@@ -47,8 +47,12 @@ jobs:
             echo "Latest matching tag: ${LATEST_TAG}"
             PATCH=$(git rev-list --count "${LATEST_TAG}..HEAD")
           else
-            echo "No matching tag found for v${MAJOR_MINOR}.*, using total commit count"
-            PATCH=$(git rev-list --count HEAD)
+            # A minor line nobody has tagged yet is a *new* line, and a new line
+            # starts at .0 — that is what "we are moving to 0.4.x" means. The
+            # old fallback here counted every commit in the repository, which
+            # would have made the first 0.4 build 0.4.234.
+            echo "No v${MAJOR_MINOR}.* tag yet — starting this line at .0"
+            PATCH=0
           fi
 
           VERSION="${MAJOR_MINOR}.${PATCH}"