fix(desktop): preserve GIF graphic control scope

[atishpatel]

Why

The GIF sanitizer removes Plain Text Extensions but previously retained their preceding Graphic Control Extensions, allowing delay, disposal, or transparency state to leak onto the next image frame.

What

• Track emitted graphic-control extensions until a rendering block consumes them
• Remove pending graphic-control state when stripping a Plain Text Extension
• Add regression coverage for a GCE followed by plain text and an image

Risk Assessment

Low — isolated to GIF metadata sanitization; clean GIFs and graphic-control extensions attached to image frames remain byte-identical.

Verification

• Built and launched the PR branch with just dev against the local relay
• Confirmed the original catjam.gif reproduces the relay 422 metadata rejection
• Ran the branch sanitizer on catjam.gif and confirmed the sanitized GIF uploads successfully as image/gif
• Downloaded the uploaded blob and verified its SHA-256 matches the relay descriptor (80f4533bd59e065dc2ad3e51dcbfa44c454dd9a61c17f651b48f9c500f7dd793)
• Verified both original and sanitized GIFs remain 48×48 with all 158 animation frames
• just desktop-tauri-test
• just desktop-tauri-clippy
• just desktop-check

References

• Follow-up to fix(desktop): strip GIF metadata extensions before upload #2425

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