Add GIF picker and drag-drop attachments

[tpmeredith]

Summary

• Adds a compose GIF picker powered by the Klipy API pattern from the local Commet/Huddle app, with backend-proxied previews so the browser does not load third-party GIF media directly.
• Adds server-side GIF download/send via the existing media send path for Google Messages, WhatsApp, and Signal.
• Adds drag-and-drop file support on the compose box, reusing the existing attachment preview/send flow.

Review Notes

• This PR targets the personal fork only. It is stacked on local/gif-drop-base, which mirrors the current local integrated main, so the diff stays focused to GIF/drop work.
• The Klipy API key is kept backend-side with an env override (KLIPY_API_KEY) and a fallback from the local Commet/Huddle implementation. If this ever goes upstream, it should likely become env-only or maintainer-configured.

Validation

• git diff --check main...HEAD
• go test ./internal/web -run 'TestGIF|TestSendGIF|TestDownloadGIFMediaRejectsNonKlipyURL|TestSendMediaUsesSignalSender|TestSendMediaUsesWhatsAppSender|TestSendMediaEndpointNoClient'\n- npm run test:e2e -- --grep 'sends a GIF through the compose picker|accepts dropped files in the compose box'\n- go test ./...\n\nLocal review artifact: .codex/reviews/20260618T044217Z-codex-gif-picker-drag-drop-a1177da.md