fix(desktop): restore DKG graph zoom controls

[branarakic-agent]

Summary

• let vertical wheel and trackpad gestures reach DKG graph canvases instead of being consumed by the WKWebView rubber-band guard
• add visible zoom-in, zoom-out, and fit-to-view controls to channel and lens graph views
• keep one initial camera fit without later settle-time fits overriding user zoom
• retain horizontal viewport-pan protection

Validation

• pnpm --dir desktop typecheck
• pnpm --dir desktop check (only pre-existing repository advisories)
• pnpm --dir desktop test — 4,759 passed
• pnpm --dir desktop build:e2e
• pnpm --dir desktop exec playwright test tests/e2e/overscroll-boundary.spec.ts tests/e2e/dkg-memory-fallback.spec.ts --project=smoke — 5 passed