fix: run ui-web page assembly on the project's pinned Python

[insuffer]

What

make build-ui assembled the served page with a bare python3, which
resolves to whatever interpreter is first on PATH. On a stock macOS
(no Command Line Tools ahead of /usr/bin/python3 = 3.9.6) the
documented self-hosting recipe make install-deps && make build-ui
died after the vite build had already succeeded:

File "ui-web/build.py", line 198, in main
    (dist / "index.html").write_text(out, encoding="utf-8", newline="")
TypeError: write_text() got an unexpected keyword argument 'newline'

Path.write_text(newline=...) exists from Python 3.10, while pyproject
requires-python is >=3.12 and every other Makefile target already
runs its Python through uv. build-ui was the one caller that bypassed
the pinned interpreter.

Change

One line in the Makefile: run the script through
uv run --frozen --python $(PYTHON_VERSION) python ui-web/build.py,
the same form install.sh already uses for this script (and already
documents the rationale for). No source, test, or dependency changes.

ui-web/build.py itself is untouched: its newline="" write needs
3.10+, but that half of the fix sits under the ui-web freeze, so this PR
takes the Makefile half only, as the issue suggests.

CI keeps its bare python3 (the ubuntu runners provide a working one
through setup-python / the runner image); this PR changes the documented
local path only.

Verification

Reproduced the failure with Python 3.9.13 (the stock-macOS analogue):

$ python3.9 ui-web/build.py
TypeError: write_text() got an unexpected keyword argument 'newline'
(exit 1)

Then ran the changed target's commands in order, on Windows with
uv 0.12.17 and Python 3.12.7:

npm ci --prefix ui-web
npm run --prefix ui-web build
uv run --frozen --python 3.12 python ui-web/build.py    -> exit 0
-> built dist/index.html (1,496,450 bytes)

Artifact checks:

sha256 ui-web/dist/index.html = a1068871929266cfe746b5f321eb7faed2158785c8b19d4984783099b4c23181
  (byte-identical to the pre-change output on 3.12)
node ui-web/scripts/check-page.mjs             -> OK (3 scripts parse)
node ui-web/scripts/count-shared-globals.mjs   -> OK
node ui-web/scripts/check-css.mjs              -> OK
node ui-web/scripts/check-class-namespace.mjs  -> OK
dist/index.html carries 0 CR bytes (LF-only artifact)

Repo gates on this commit:

scripts/check_source_language.py HEAD~1..HEAD  -> pass
scripts/check_commit_messages.py HEAD~1..HEAD  -> pass

Based on main at 87f3928.

Closes #526

AI disclosure: this PR was drafted with AI assistance (ZCode agent,
model step-5-preview); the reproduction, commands, and results above
were produced by it. The diff is one line; please review before merging.

[insuffer]

Hi — first contribution here: a one-line fix so the ui-web page assembly runs on the project's pinned Python instead of whatever is first on PATH. The Commit lint, CI, and Docs workflow runs are waiting for fork approval — could a maintainer approve them so the checks can run? Thanks!