Add support for choosing high-mem variant for Colab Pro users

[Copilot]

• Analyze existing code structure for server selection flow
• Add shapeToRamType helper function in api.ts to map Shape enum to human-friendly name
• Update ColabServerDescriptor interface to include optional shape property
• Update ColabClient.assign() and related methods to pass shape parameter in assignment URL
• Modify ServerPicker to prompt for RAM type (Standard/High) when user is Pro subscriber
• Update AssignmentManager.assignServer() to pass shape to client
• Add unit tests for shape selection in ServerPicker
• Add unit tests for shape parameter in ColabClient.assign()
• Identify user's subscription tier and call prompt() with true for Pro/Pro+ users
• Add unit tests for high-mem eligibility based on subscription tier
• Build and verify all tests pass (439 tests passing)
• Code review completed
• Security scan completed - no vulnerabilities found

Original prompt

Add support for choosing a high-mem variant for colab pro users.

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[corepattern]

@copilot Identify the user's tier and call prompt() with True if the user is pro or above

[Copilot]

@copilot Identify the user's tier and call prompt() with True if the user is pro or above

Updated assignServer() in provider.ts to check the user's subscription tier before calling prompt(). Users with PRO or PRO_PLUS tier will now see the RAM selection step. Added 4 tests to verify this behavior. (bd91d12)