feat: show the installer demo on readme and site

The README shows the GIF under the commands. The home page and Get your
files show the video through InstallDemo.astro: nothing is fetched until
it is on screen, reduced motion and Save-Data keep the poster with a play
button, and a description is read to screen readers.

scripts/generate_readme.py

@@ -440,6 +440,9 @@ def format_size(size: int) -> str:
 
 
 SITE_URL = "https://abdess.github.io/retrobios/"
+# The installer demo (scripts/make_demo.py): the recording and what it shows.
+DEMO_DIR = Path(__file__).resolve().parent.parent / "docs_assets" / "demo"
+DEMO_GIF = "docs_assets/demo/install.gif"
 REPO_PATH = "../.."
 REPO = "Abdess/retrobios"
 # GitHub's "Add file > Upload files" page for bios/: a browser upload by
@@ -447,6 +450,19 @@ REPO = "Abdess/retrobios"
 UPLOAD_URL = f"https://github.com/{REPO}/upload/main/bios"
 
 
+def demo_facts() -> dict:
+    """What the installer demo shows, as make_demo.py measured it."""
+    return json.loads((DEMO_DIR / "demo.json").read_text(encoding="utf-8"))
+
+
+def demo_description(facts: dict | None = None) -> str:
+    """The demo in words, for a reader who cannot see it."""
+    facts = facts or demo_facts()
+    return (f"Recording of the one-line command in a terminal. The installer finds an "
+            f"{facts['setup']} SD card with {facts['games']} games, plans {facts['files']} "
+            f"BIOS files ({facts['size']}) for them, installs them and shows the summary.")
+
+
 def _existing_contributor_block(readme_path: str = "README.md") -> list[str]:
     """The contributors already published, read back from the file.
 
@@ -593,6 +609,8 @@ def generate_readme(db: dict, platforms_dir: str, db_path: str = "database.json"
         install_command("powershell", "--check"),
         "```",
         "",
+        f"![{demo_description()}]({DEMO_GIF})",
+        "",
         "The script opens a full-screen installer, driven with the arrow keys and Enter, or with a"
         " controller on a handheld. It finds your setup and its BIOS folder, downloads only the missing"
         " files and checks each one. It can also copy the BIOS files from your old SD card.",

scripts/generate_site.py

@@ -82,6 +82,8 @@ from generate_readme import (
     NO_PACK,
     asset_links,
     contributing_text,
+    demo_description,
+    demo_facts,
     download_links,
     extract_notes,
     extract_targets,
@@ -555,6 +557,7 @@ def generate_home(
             "**Run one command.** Some setups have a ZIP.",
             "**Start a game.**",
         ]),
+        *install_demo(),
         "## Pick your device",
         "",
         *setup_picker(registry, display),
@@ -4188,6 +4191,20 @@ def first_screen_words() -> list[str]:
             f"and **{installer.TUI_FOLDER_TITLE}**), then offers **{installer.TUI_INSTALL_ROW}**."]
 
 
+# The demo files the site serves (docs_assets/demo/, scripts/make_demo.py):
+# the GIF is the README's, the tape and demo.json are inputs.
+DEMO_ASSETS = ("install.webm", "install.mp4", "install.png")
+
+
+def install_demo() -> list[str]:
+    """The installer demo: a video that loads once it is on screen, its
+    description for screen readers."""
+    facts = demo_facts()
+    text = html.escape(demo_description(facts), quote=True)
+    return [f'<InstallDemo description="{text}" width="{facts["width"]}" '
+            f'height="{facts["height"]}" />', ""]
+
+
 def check_pass_line() -> str:
     """The pass line of "How to check": the words `--check` prints only when
     no file is missing or wrong (install.py EVERYTHING_IN_PLACE), read from
@@ -12117,6 +12134,7 @@ def generate_which_pack() -> str:
         "",
         *first_screen_words(),
         "",
+        *install_demo(),
         *trust_box(""),
         "**Check your folder later.** Add `--check`. It changes nothing.",
         "",
@@ -12476,6 +12494,10 @@ def _generate(args: argparse.Namespace) -> None:
         source = Path("docs_assets") / folder
         if source.is_dir():
             shutil.copytree(source, public / "assets" / dest, dirs_exist_ok=True)
+    demo_dest = public / "assets" / "demo"
+    demo_dest.mkdir(parents=True, exist_ok=True)
+    for name in DEMO_ASSETS:
+        shutil.copy2(Path("docs_assets") / "demo" / name, demo_dest / name)
     assets_src = Path(".github") / "assets"
     for name, dest_name in [("logo.png", "logo.png"), ("favicon.png", "favicon.png")]:
         src = assets_src / name

scripts/sitestarlight.py

@@ -29,6 +29,7 @@ COMPONENTS = {
     "TabItem": "@astrojs/starlight/components",
     "FilePicker": "components/FilePicker.astro",
     "MeasurementsExplorer": "components/MeasurementsExplorer.astro",
+    "InstallDemo": "components/InstallDemo.astro",
 }
 
 # Pages of file cards: level 3 headings by the hundred, so the table of