feat: add Giphy GIF provider with selectable provider and in-app API keys

[dgshue]

Problem

GIF search is hardcoded to Tenor and depends on a build-time TENOR_API_KEY. Users building from source — or who prefer Giphy — can't choose a provider or supply their own key without rebuilding.

Solution

• New gifProvider setting ("tenor" | "giphy" | "none", defaults to "tenor" — existing behavior unchanged)
• New tenorApiKeyOverride / giphyApiKeyOverride settings to paste your own keys at runtime (falls back to the compiled-in key when blank)
• New GIF Picker section in Settings → Message View to choose the provider and manage keys
• New showApiKeyDialog() with obscured input + provider sign-up link
• The text-field GIF button routes to Tenor or Giphy based on the selection
• Adds giphy_get: ^3.6.1+1

No changes to existing Tenor behavior when defaults are kept.

Testing

Built and manually tested on Windows desktop:

• Switching the GIF provider between Tenor and Giphy
• Entering and clearing API keys via the new dialog
• Searching and inserting GIFs from both providers

Screenshots

N/A — happy to add on request.

[jjoelj]

We are likely dropping default in-app GIF keyboard support in favor of gif insertion from the win+. menu, but will consider this for people who want to bring their own keys.