Surf 0.16.0: shared client core, Linux preview and native UI

[seg6]

Summary

Prepare Surf 0.16.0: extract the shared C99 client core, ship a usable Linux
client preview, repair downloads across clients/backend, restore native iOS
controls and improve dark appearance, and align the dashboard and repository
page in light/dark mode.

The dashboard appearance picker is in the footer; the Get iOS app button is
removed. Compatibility generation remains 1. Existing installations update in
place without uninstalling or resetting data.

Validation

• make test (portable core, all Go packages, Rust workspace)
• make client-core-sanitize
• go test -race ./...
• Rust formatting and workspace Clippy with warnings denied
• Real X11 desktop input integration
• Secure-session / FFmpeg / OpenGL integration, 600 presented frames
• Universal iOS 6/7 package build and verification
• Cydia repository build and checksum/template verification
• User reports iPad behavior tested and working; user requested to own dashboard
  visual review. Dashboard scripts parse and markup regressions are tested.

The backend CI job now runs the Go suite directly; C and Rust remain covered by
their own jobs with the required dependencies.

CI now declares the headless X11/media tools and clocked virtual audio device,
uses Node 24 actions, retains compiled Rust dependencies after test failures,
separately caches debug/release targets, and caches the native toolchain Docker
layers. Verified main builds warm release dependencies before a signed tag.
Functional streaming checks retain frame/deadline/queue/error assertions;
SURF_TEST_MIN_FPS=55 enables the hardware benchmark on a controlled machine
(latest local run: 58.17 FPS, 600 frames, zero decode errors).