Add Giphy ASCII animation widget

[muk2]

Summary

Adds a new widget that fetches GIFs from the Giphy API, decodes them frame-by-frame, converts each frame to ASCII art, and animates them directly in the terminal.

• Giphy API integration (src/feeds/giphy.rs): Supports three modes — search (query-based), trending, and random. Fetches the small fixed-height variant for optimal terminal rendering. Downloads the GIF binary and decodes all frames.
• ASCII frame conversion: Each GIF frame is decoded to RGBA pixels, downsampled to fit terminal dimensions (max 60×20), converted to grayscale via luminance weighting (0.299R + 0.587G + 0.114B), then mapped to an ASCII character gradient: " .:-=+*#%@".
• Animation widget (src/ui/widgets/giphy.rs): Renders ASCII frames sequentially with configurable frame delay (default 150ms). Supports pause/resume and manual frame stepping via scroll keys. Shows frame counter and GIF title in the border.
• Configuration via config.toml:

  [[widgets]]
  type = "giphy"
  title = "GIF Player"
  api_key = "YOUR_GIPHY_API_KEY"
  query = "cats"
  mode = "search"          # search | trending | random
  frame_delay_ms = 150
  position = { row = 0, col = 2 }

Files changed

File	Change
Cargo.toml	Add gif crate dependency
src/feeds/giphy.rs	New — Giphy API fetcher + GIF→ASCII decoder
src/feeds/mod.rs	Add Giphy variant to FeedData, GiphyGif struct
src/ui/widgets/giphy.rs	New — Animated ASCII art widget
src/ui/widgets/mod.rs	Register giphy module
src/config.rs	Add GiphyConfig struct + Giphy widget variant
src/app.rs	Wire up widget creation + animation tick loop

Performance considerations

• Frames capped at 30 per GIF to prevent memory issues
• Terminal output sized to max 60×20 characters regardless of source resolution
• Frame delay configurable to prevent excessive redraws

Test plan

• CI passes (check, fmt, clippy, build on all platforms)
• Widget renders loading state before GIF is fetched
• Widget renders error state when API key is invalid
• ASCII animation plays smoothly with correct frame timing
• Scroll up/down manually steps through frames
• Widget resizes correctly with terminal

Closes #55

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