Add an intent to import a brush directly to Brush Graph

[maxmmitchell]

Adds the com.example.cahier.intent.action.IMPORT_BRUSH intent, which takes a uri as an input, and:

• Attempts to decode it as a BrushFamily
• If successful, loads the new BrushFamily into the active brush slot and the __autosave__ slot in the custom brush store
• Navigates to Brush Graph for immediate editing of this brush

This is useful for workstation custom brush creation, to enable quickly sending a .brushfamily file created in a developer's workstation over to Cahier for rapid iteration. As we explore new flows for brush design, this intent is a helpful utility.

Note that the 10 MB limit for .brushfamily files is arbitrary, but should be more than enough (Shading Pencil is our largest annotated brush, and is 373 KB -- most are under 1 KB).

[gemini-code-assist]

Code Review

This pull request introduces the ability to import custom brushes from a URI via a new intent filter. It updates MainActivity to handle the import intent, decode and save the brush, and navigate to the Brush Graph screen. The review feedback highlights a bug where configuration changes trigger duplicate imports, a potential OutOfMemoryError vulnerability when reading unlimited stream bytes, and an ignored return value when loading the imported brush family.

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