Add presentation slideshow, expand docs examples

[jlowin]

Adds a presentation slideshow example that exercises the new key bindings feature, a set of mini examples for blog embeds, and a broad expansion of the docs examples gallery. Also fixes two previews that were silently broken.

Presentation (examples/presentation/): fleet_performance.py (DataTable with progress gauges), cost_overview.py (pie + area chart), and slideshow.py (stitches them with arrow-key nav, prev/next buttons, and a Shift+? shortcuts dialog).

Mini examples (examples/mini/): hello_world, reactive_binding, dynamic_list, conditional, actions — small self-contained PrefabApps for blog embeds. Removes the old loose dashboard.py/growth.py/etc.

Docs examples gallery: seven new pages — actions, conditional, deploy-pipeline, dynamic-list, expense-tracker, reactive-binding, server-dashboard — and a restructured nav (Interactivity / Data & Visualization / Apps). Updates contract fixtures for Dialog.dismissible and Kbd.

Broken-preview fixes:

• cross-filter referenced metrics_north, metrics_all, etc. in a let binding but never loaded them into state — the dashboard rendered empty. Wraps the layout in a PrefabApp(state=...) with flattened per-region keys.
• server-dashboard passed variant names into Metric.delta, which rendered as neutral "— success" / "— stable" badges. Drops the bogus deltas, pins the app container to max-w-md to stop it from flexing width each tick, and makes Uptime/Requests/p99 Latency reactive.

# cross-filter: state is now flattened and wired through the let binding
state = {"region": "all"}
for prefix, data in (("metrics", metrics), ("monthly", monthly), ...):
    for region in REGIONS:
        state[f"{prefix}_{region}"] = data[region]

with PrefabApp(state=state, css_class="max-w-none p-0"):
    with Dashboard(columns=12, let={"m": pick("metrics"), ...}):
        Metric(label="Revenue", value=m.revenue.currency())