webui: make the committed bundle reproducible

[christopherthompson81]

Fixes #545.

dist/index.html is committed and embedded into the server binary at configure time (CMakeLists.txt:2226) — that is what lets the project build a working server without a JavaScript toolchain. But it is not reproducible: SvelteKit defaults kit.version.name to Date.now() and derives the embedded __sveltekit_<id> from it.

	build 1	build 2
before	__sveltekit_1ia7gsf / bb3f9c3c…	__sveltekit_1xqgjp8 / 6cd91a94…
after	__sveltekit_1vzi1g / 65bed68b…	__sveltekit_1vzi1g / 65bed68b…

So any two branches that rebuild the web UI conflict in that file regardless of whether their source overlaps. That is what happened to #539, where dist/index.html was the only conflict while src/lib/text.ts merged cleanly and no commit on main had touched either file.

Uses the package version rather than a constant: stable for a given source tree, so the bundle reproduces, but it changes when the version is bumped, so SvelteKit's client-side "app has been updated" check keeps working across releases. A hard-coded string would have disabled that silently.

Verification

The embedded id is a pure function of this setting. Building current main with the timestamp its committed bundle carries (1789350153403) produced __sveltekit_ega6lw — matching it — and a file byte-identical to the committed one. So this toolchain reproduces what is in the repo, and the setting is the only thing steering the id.

Nothing else in the UI changes. dist/index.html is rebuilt here so the tree is consistent — after this, npm run build leaves git status clean instead of producing a diff on every run. The rebuilt bundle differs from the committed one only in the version constant and the two things derived from it (the SvelteKit id, and the Vite content hashes, since the version string lives inside a chunk). Normalising those three leaves zero differing lines.

Alternatives considered

Stop committing dist/. This was the first idea and it has a real cost: the CMake step above falls back to a placeholder page when the file is absent, and the fallback succeeds. Removing the bundle would mean either requiring node for every build, or quietly shipping a server whose UI is a placeholder for anyone who did not run npm run build first. Making the artifact deterministic fixes the conflicts without giving that up.

.gitattributes with -merge on the file would stop git attempting a line-merge of minified output. Conflicts would still occur, but resolve as "take either, then rebuild" rather than producing a corrupted hybrid. Worth doing as well, perhaps, but it treats the symptom.

[christopherthompson81]

Moving this to draft so it isn't sitting in your review queue while you think over the trade-offs in #545 — no pressure either way, just ping me and I'll mark it ready again.

[0xShug0]

@mirek190 Any thoughts on this PR?