fix(auth): use backend project ID for free tier Gemini CLI OAuth users

Fixes issue where free tier users cannot access Gemini 3 preview models
due to frontend/backend project ID mapping.

## Problem
Google's Gemini API uses a frontend/backend project mapping system for
free tier users:
- Frontend projects (e.g., gen-lang-client-*) are user-visible
- Backend projects (e.g., mystical-victor-*) host actual API access
- Only backend projects have access to preview models (gemini-3-*)

Previously, CLIProxyAPI ignored the backend project ID returned by
Google's onboarding API and kept using the frontend ID, preventing
access to preview models.

## Solution
### CLI (internal/cmd/login.go)
- Detect free tier users (gen-lang-client-* projects or FREE/LEGACY tier)
- Show interactive prompt allowing users to choose frontend or backend
- Default to backend (recommended for preview model access)
- Pro users: maintain original behavior (keep frontend ID)

### Web UI (internal/api/handlers/management/auth_files.go)
- Detect free tier users using same logic
- Automatically use backend project ID (recommended choice)
- Pro users: maintain original behavior (keep frontend ID)

### Deduplication (internal/cmd/login.go)
- Add deduplication when user selects ALL projects
- Prevents redundant API calls when multiple frontend projects map to
  same backend
- Skips duplicate project IDs in activation loop

## Impact
- Free tier users: Can now access gemini-3-pro-preview and
  gemini-3-flash-preview models
- Pro users: No change in behavior (backward compatible)
- Only affects Gemini CLI OAuth (not antigravity or API key auth)

## Testing
- Tested with free tier account selecting single project
- Tested with free tier account selecting ALL projects
- Verified deduplication prevents redundant onboarding calls
- Confirmed pro user behavior unchanged

internal/api/handlers/management/auth_files.go

@@ -2102,7 +2102,20 @@ func performGeminiCLISetup(ctx context.Context, httpClient *http.Client, storage
 			finalProjectID := projectID
 			if responseProjectID != "" {
 				if explicitProject && !strings.EqualFold(responseProjectID, projectID) {
-					log.Warnf("Gemini onboarding returned project %s instead of requested %s; keeping requested project ID.", responseProjectID, projectID)
+					// Check if this is a free user (gen-lang-client projects or free/legacy tier)
+					isFreeUser := strings.HasPrefix(projectID, "gen-lang-client-") ||
+						strings.EqualFold(tierID, "FREE") ||
+						strings.EqualFold(tierID, "LEGACY")
+
+					if isFreeUser {
+						// For free users, use backend project ID for preview model access
+						log.Infof("Gemini onboarding: frontend project %s maps to backend project %s", projectID, responseProjectID)
+						log.Infof("Using backend project ID: %s (recommended for preview model access)", responseProjectID)
+						finalProjectID = responseProjectID
+					} else {
+						// Pro users: keep requested project ID (original behavior)
+						log.Warnf("Gemini onboarding returned project %s instead of requested %s; keeping requested project ID.", responseProjectID, projectID)
+					}
 				} else {
 					finalProjectID = responseProjectID
 				}