engine.go: use whatsmeow as the call control plane

[purpshell]

Rebuilds the media-backend split on current meowcaller main and the refreshed tulir/whatsmeow#1201 API.

Changes:

• delegates signaling, call-key exchange, direction-aware relay election, mute, and video state stanzas to whatsmeow
• preserves meowcaller RTP/SRTP, MLow, H.264 framing, reactions, diagnostics, independent video flows, and answering-device rekeying
• removes the duplicated signaling package and raw-node/unsafe hooks
• exposes Call.SetMuted through the whatsmeow control plane
• publishes the standalone browser console in examples/web with QR pairing, audio/video calls, reactions, dynamic capture dimensions, and stable portrait rendering
• keeps examples/cli focused on terminal audio workflows

Dependency:

• replaces go.mau.fi/whatsmeow with github.com/titan-api/whatsmeow at edfef31 until call: add first-class 1:1 call signaling and media-handoff API tulir/whatsmeow#1201 merges

Validation:

• GOPROXY=direct go test ./...
• GOPROXY=direct go vet ./...
• GOPROXY=direct go test -race .
• examples/cli: GOPROXY=direct go test ./... and go vet ./...
• examples/web: GOPROXY=direct go test ./... and go vet ./...
• browser console checked at desktop and 375px mobile widths with no console errors or horizontal overflow