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-preview.yml

@@ -54,7 +54,21 @@ jobs:
         run: |
           MAJOR_MINOR=$(cat VERSION | tr -d '[:space:]')
           SHORT_SHA=$(git rev-parse --short HEAD)
-          VERSION="${MAJOR_MINOR}.0-preview.${SHORT_SHA}"
+
+          # The patch number is computed exactly as build-app.yml does it, so a
+          # preview is labelled with the version the release it previews would
+          # carry. This used to be hard-coded `.0`, which made every preview
+          # installer claim to be x.y.0 no matter what it contained.
+          LATEST_TAG=$(git tag -l "v${MAJOR_MINOR}.*" --sort=-v:refname | grep -E "^v${MAJOR_MINOR}\.[0-9]+$" | head -1 || true)
+          if [ -n "$LATEST_TAG" ]; then
+            PATCH=$(git rev-list --count "${LATEST_TAG}..HEAD")
+            echo "Latest matching tag: ${LATEST_TAG} (+${PATCH} commits)"
+          else
+            echo "No v${MAJOR_MINOR}.* tag yet — starting this line at .0"
+            PATCH=0
+          fi
+
+          VERSION="${MAJOR_MINOR}.${PATCH}-preview.${SHORT_SHA}"
           echo "VERSION=${VERSION}" >> $GITHUB_OUTPUT
           echo "Computed preview version: ${VERSION}"