fix(proxy): account-aware Gemini Pro remapping with dynamic fallback

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Summary

• keep concrete Gemini Pro model IDs as pass-through in built-in mapping (no forced cross-version rewrite)
• add account-aware dynamic model rewrite fallback for Gemini 3/3.1 Pro family
• apply dynamic rewrite in both OpenAI-compatible and Gemini-native handlers
• document current remapping behavior in docs/model-remapping-logic.md

Why

• request model should be preserved when that model is available for the selected account
• fallback remapping should only happen when the requested model is not available

Files

• src-tauri/src/proxy/common/model_mapping.rs
• src-tauri/src/proxy/token_manager.rs
• src-tauri/src/proxy/handlers/openai.rs
• src-tauri/src/proxy/handlers/gemini.rs
• src-tauri/src/proxy/mappers/common_utils.rs
• docs/model-remapping-logic.md

Validation

• cargo check passed
• local smoke request confirmed gemini-3-pro-high remains gemini-3-pro-high when available