add dashboard train page and AI Studio flow

[GauravOP-03]

📝 Description

Add the dashboard Train AI (AI Studio) page and all related modified/added files: the page, hooks (useAITraining, useChannelVideos), and train-specific components (header, skeleton, video grid, training progress). By default, returning users see only their previously trained videos instead of refetching from the YouTube API; the API is called only when they explicitly click Train AI Again, which reduces YouTube API usage. No other app changes.

🎯 Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🎨 Style/UI changes (formatting, missing semi colons, etc; no logic change)
• ♻️ Refactoring (no functional changes, code improvements)
• ⚡ Performance improvements
• 🧪 Adding or updating tests
• 🔧 Configuration changes

🧪 Testing

• I have tested my changes locally
• I have added tests for my changes
• All existing tests pass
• I have tested on different browsers/devices (if applicable)

Describe: With YouTube connected, open /dashboard/train, select 3–5 videos, start training; confirm progress and success dialog. With YouTube disconnected, confirm locked overlay. For an already-trained user, confirm “Trained Videos” view by default and that “Train AI Again” triggers channel video fetch.

📋 Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have checked my code and corrected any misspellings
• I have read the Contributing Guide

📸 Screenshots (if applicable)

🔧 Technical Details

Files in this PR (only these):

File	Change
app/dashboard/train/page.tsx	New. Renders TrainAIHeader, left-column StepperGuide (“How does model training work?”), right-column ChannelVideoGrid or TrainingProgress from useAITraining. Shows SuccessDialog with next steps when training completes.
hooks/useAITraining.ts	Modified/New. Profile, selection, training state; calls /api/v1/youtube/trained-videos and /api/v1/train-ai; useSSE for job status; exposes handleStartTraining, handleToggleVideo, handleConnectYoutube, progress, showModal, etc.
hooks/useChannelVideos.ts	Modified/New. Fetches channel videos (paginated); exposes videos, loading, loadingMore, hasMore, fetchVideos, loadMore. Exports ChannelVideo type. Used by ChannelVideoGrid and useAITraining.
train/TrainAIHeader.tsx	New. Title “AI Studio”, subtitle, status pills: YouTube Connected/Pending, Model Trained/Untrained, credits used (when trained).
train/skeleton/TrainAIPageSkeleton.tsx	New. Skeleton for header + two-column layout (guide + card) while profile loads.
train/ChannelVideoGrid.tsx	New. Uses useChannelVideos. “Select Training Data” vs “Trained Videos” with “Train AI Again” toggle; search; grid of VideoCard (select 3–5); Start Training / Submit New Videos; load more; locked overlay with “Connect Channel” when not connected. YouTube API: After the user has trained once, the grid shows the cached “Trained Videos” list (from youtube_channels.youtube_trained_videos / /api/v1/youtube/trained-videos). Channel videos are fetched only when the user clicks “Train AI Again”, avoiding unnecessary API calls for users who are just viewing the page or their trained set.
train/TrainingProgress.tsx	New. Wraps GenerationProgress with train steps (Queued → Analyzing → Training → Saving → Done) and hint.

Dependencies (must exist elsewhere): SuccessDialog, StepperGuide, GenerationProgress, api-client, useSSE, UI components.

🚀 Deployment Notes

• Backend /api/v1/train-ai and /api/v1/youtube/trained-videos (and YouTube OAuth) must be available.

📚 Documentation Updates

• README.md updated
• CONTRIBUTING.md updated (if applicable)
• API documentation updated (if applicable)
• Component documentation updated (if applicable)

🔍 Review Notes

• Confirm flow: connect YouTube → select videos → start training → progress → success dialog.
• Confirm trained users see “Trained Videos” by default and “Train AI Again” triggers channel fetch.
• Confirm trained-videos persist and retrain flow works.

📊 Performance Impact

• No performance impact
• Performance improvement
• Performance regression (explain below)

Details: Fewer YouTube API calls. Previously the page could trigger channel video fetches even when the user did not intend to retrain. Now trained users see cached trained videos by default; channel videos are fetched only when they click “Train AI Again”, saving quota and load time.

🔒 Security Considerations

• No security implications
• Security improvement
• Potential security concern (explain below)

Details: Train and channel endpoints are called with requireAuth: true.

🎉 Additional Notes

• This PR is limited to the train page and the files listed above.
• Defaulting to “Trained Videos” for already-trained users reduces YouTube API usage compared to always loading the full channel video list.

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