feat: add KaTeX math and Gemini image generation

[JOAO2666]

Summary

• Fixes Allow selecting multiple images in a single picker action #5: selecting multiple images from one Android picker action.
• Adds Portuguese (Brazil) labels for the new controls.
• Renders Markdown math locally with bundled KaTeX assets, including inline and display LaTeX.
• Adds Gemini image generation through the native Gemini API, with an optional reference image.
• Adds a settings action that fetches image-capable Gemini models available to the configured API key.
• Adapts image-generation cancellation and completion to the current activity-lifecycle generation state.

Security and scope

• No API key, token, or credential is committed.
• The generated debug APK is intentionally not in this pull request. It is available only in the contributor fork's release branch, so this PR remains source-reviewable.

Validation

• Built :app:assembleDebug successfully with JDK 8.
• Confirmed the APK package as io.github.gohoski.numai, version 1.0.1 (versionCode 4).
• Device validation on HW2 Ultra has not been performed by this change; this remains needed before calling the hardware flow verified.

Closes #5