Portable offline WASM export for the fit app and docs explorers

[marcocaggioni]

Makes the browser fit app and all nine documentation explorers boot with no network access.

What changes

• marimo export html-wasm --offline for app/fit_app.py (deploy-app.yml) and all nine explorers (docs/export_interactive.py): the Python runtime, Pyodide, and every dependency wheel — including rheofit and its SciPy/pandas/matplotlib/openpyxl stack for the fit app — now ship with the page. Nothing is fetched from a package index at runtime.
• The nine explorers share one runtime: the identical assets/pyodide/packages/lockfile directories are moved to docs/_static/interactive/_shared/ and each explorer page references them relatively. One ~59 MB runtime instead of nine (~530 MB saved per docs build).
• Build environments get what the offline export needs: headless Chromium via Playwright in deploy-app.yml and the docs CI build job, plus the Chromium system libraries and a playwright install chromium step in .readthedocs.yaml (marimo>=0.25 in docs/requirements.txt).

Verification

• Fit app: served locally with every non-localhost request blocked — reaches the upload screen (title, Data upload, model picker, Compare section), zero external requests, zero page errors.
• TC explorer in the shared layout: full working screen — live sliders, parameter readouts, and the log-log stress/viscosity plot with decomposition lines — with all external requests blocked, zero external requests, zero page errors.

Notes for review

• The previous online export commands are one revert away in git history until this merges; nothing else in the app or docs changes.
• Deploy size grows (~77 MB for the app, ~59 MB shared runtime for the docs) since wheels now ship with the pages.
• Please merge from the GitHub UI when ready.