Add retry logic, network resilience, and missing image sizes UI for media uploads

[adamsilverstein]

Summary

Enhance client-side media processing with three reliability improvements:

• Automatic retry with exponential backoff — Failed uploads should be retried up to 3 times with increasing delays (1s → 2s → 4s) and jitter to prevent thundering herd
• Network resilience — Upload queue should pause when the browser goes offline and resume automatically when connectivity returns
• Missing image sizes UI — When images have incomplete sub-sizes (e.g., from interrupted uploads), the editor should show visual indicators and let users generate them on demand

Missing Image Sizes — User Flow

Instead of auto-generating missing sizes silently on editor load, give users control:

1. Info icon on images — A small indicator appears on images with missing sub-sizes
2. Popover with Generate action — Clicking the icon shows details and a link to trigger generation
3. Pre-publish check — The "Are you ready to publish?" panel warns about missing sizes with a bulk Generate option

Retry Flow

flowchart LR
    A[Upload] --> B[Processing]
    B --> C{Success?}
    C -->|Yes| D[Complete]
    C -->|No| E{Retryable?}
    E -->|No| F[Failed]
    E -->|Yes| G{Attempts < 3?}
    G -->|No| F
    G -->|Yes| H[Wait with backoff]
    H -->|1s / 2s / 4s| B

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Network Resilience

flowchart LR
    A[Upload Queue] --> B{Browser Online?}
    B -->|Yes| C[Continue Processing]
    B -->|No| D[Pause Queue]
    D --> E[Wait for reconnect]
    E -->|online event| F[Resume Queue]
    F --> C
    C --> G[Complete]

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Acceptance Criteria

• Failed uploads retry up to 3 times with exponential backoff
• Upload queue pauses when offline and resumes on reconnect
• Info icon appears on images with missing sub-sizes
• Clicking icon shows popover with Generate link
• Generate creates missing sizes and hides indicator
• Pre-publish panel shows Missing image sizes suggestion
• Generate in pre-publish processes all missing sizes with spinner

PR: #76765