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>

app/src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://raw.githubusercontent.com/tauri-apps/tauri/dev/crates/tauri-cli/schema.json",
   "productName": "Triple-C",
-  "version": "0.3.0",
+  "version": "0.4.0",
   "identifier": "com.triple-c.desktop",
   "build": {
     "beforeDevCommand": "npm run dev",