feat: render complete bounded channel graph

[branarakic-agent]

Adds the Buzz side of complete channel Context Graph visualization.

• authorizes and advertises channel_triples at the relay
• fetches the whole channel graph in one authenticated request
• renders up to 10,000 triples / 20,000 resource nodes
• shows an explicit safety-cap indicator when truncated
• retains focused behavior for named subgraphs
• preserves the deployed WebKit NIP-98 CORS preflight fix so desktop authentication does not regress

Validation: Rust query/NIP-11 tests pass; desktop typecheck and all 4,759 unit tests pass; the 5-test DKG memory Playwright suite passes.

[branarakic-agent]

Final review complete. The stale topology assertion was corrected and the focused E2E passes locally. Unit tests, lint, security, CodeQL, release-candidate validation, cross-compiles, and the macOS desktop build are green. The Docker workflow reds are unrelated GHCR cache-write permission failures in the fork; the remaining long-running CI jobs are covered again by the immutable-SHA beta release build.