Decide whether the theme checks a notebook exists before showing Launch

[quantecon-services]

Split out of #205, whose acceptance criteria allow this part to be "either implemented as decided, or split into its own issue before this one closes". #205 ships the opt-in configuration change; this issue carries the deferred failsafe.

What #205 already fixes

Launch now renders only when launch_notebook_repo names a notebook repository and launch_colab is true, and nothing is derived from project.github. That removes every dead link we can currently point at:

• lecture-wasm and the four unconfigured translations (lecture-python-programming.fa, .fr, .zh-cn, .ml) have no notebooks repository, so they now show no Launch at all rather than linking to a guessed <source>.notebooks that 404s.
• lecture-dp, lecture-python.zh-cn, lecture-intro.zh-cn name a notebooks repository that does not exist; those values are being removed Sphinx-side (Drop nb_repository_url, which points at a notebooks repo that does not exist lecture-dp#60, Drop nb_repository_url, which points at a notebooks repo that does not exist lecture-python.zh-cn#279, Drop nb_repository_url, which points at a notebooks repo that does not exist lecture-intro.zh-cn#305), and at cutover they simply do not set launch_notebook_repo.

What is left

A narrower case: a site whose notebooks repository does exist, but where an individual page's notebook is missing from it — or where the repository is renamed or emptied after the build. The maintainer wants a failsafe so Launch never opens a missing notebook.

Three approaches were weighed on #205; the current thinking is the first.

Approach	How it works	For	Against
In-page failsafe (current thinking)	Launch starts hidden. A script requests the page's notebook (e.g. HEAD https://raw.githubusercontent.com/<repo>/<branch>/<path>.ipynb) and reveals the control only on success. raw.githubusercontent.com allows cross-origin requests and 404s for a missing repository (checked 2026-09-11)	Accurate per page; catches a repository renamed or emptied after the build; no build change	One request to GitHub per page view (cacheable for the session); the control appears after load; readers without JavaScript, or where GitHub is blocked, never see it
Build-time check	The theme fetches the notebook repository's file list once per build and renders Launch only on pages whose notebook is listed	Static, repeatable output; no requests from readers	Needs network during the build and a GITHUB_TOKEN in CI (the unauthenticated GitHub API allows 60 requests an hour); goes stale if the repository changes after the build
Notebooks served from the site	Notebooks are published with the site (QuantEcon/quantecon-book-theme#359, or the fork's ipynb export), so the build knows which exist	No check needed, and no companion repositories	Relies on Colab opening a notebook by URL (claimed in QuantEcon/quantecon-book-theme#359, not verified), and on every site publishing its notebooks

Whichever is chosen:

• Fail closed — hide the control on a timeout or error.
• Keep the toolbar steady — no layout shift and no empty slot. Launch control cannot be turned off and links to a nonexistent notebooks repo on sites without one #205 gives the slot a qe-launch-slot class and empty:hidden, so a hidden control already collapses rather than leaving a gap.
• Decide per page.
• Test the hidden state.

Acceptance criteria

• The approach is decided and recorded on this issue.
• Launch is not shown on a page whose notebook cannot be found, and the toolbar takes no gap in its place.
• The control fails closed: a timeout, a network error or a blocked request leaves it hidden.
• The hidden state is covered by a test.
• If the in-page approach is chosen, the docs say that readers without JavaScript see no Launch control.

Context

• Parent decision: Launch control cannot be turned off and links to a nonexistent notebooks repo on sites without one #205's decision comment, section "Deferred: checking the notebook exists before Launch is shown".
• Not on the Book-theme parity cutover — tracking #147 gate unless the maintainer adds it: Launch control cannot be turned off and links to a nonexistent notebooks repo on sites without one #205 closes the gap that gate item was raised for.
• Related: QuantEcon/workspace-lectures#12 (notebook sync, which records that lecture-dp has no notebooks repository) and Update Colab badge URLs to support notebooks on gh-pages quantecon-book-theme#359 (notebooks served from the site).