fix(example): bound multimodal xkcd requests

[Ghraven]

Problem

examples/multimodal-generate.py fetches xkcd metadata and the comic image using httpx.get() without explicit timeouts. If xkcd or the image host accepts a connection but stalls, the example can hang before it ever reaches the Ollama generation call.

Before / after behavior

Before, the example made three external HTTP requests without a local timeout at the call sites.

After, the example uses one small HTTP_TIMEOUT constant for the latest-comic lookup, selected-comic lookup, and image download. The existing raise_for_status() behavior is unchanged.

Verification

• python -m ruff format examples/multimodal-generate.py
• python -m ruff check examples/multimodal-generate.py
• python -m py_compile examples/multimodal-generate.py
• git diff --check

I did not run the example end-to-end because it requires a local Ollama llava model and live network calls.