Multi-element selection per capture session

[BraedenBDev]

Currently each capture session captures a single selected element. Many real-world feedback scenarios involve multiple elements:

• "These three buttons have inconsistent padding"
• "The spacing between this header and this card is off"
• "This nav item and this breadcrumb should match"
• "Compare these two sections — one has the old style"

Proposed behavior:

• After selecting the first element, the selector stays active (or can be re-activated)
• Each click adds another element to the session's selection array
• Each selected element gets its own timestamp, CSS selector, computed styles, and optional component info
• The sidepanel shows a list of selected elements with visual indicators (numbered badges?)
• The compiled output includes all selected elements in order

Data model change:
```typescript
// Current
interface CaptureSession {
selectedElement?: ElementInfo;
}

// Proposed
interface CaptureSession {
selectedElements: ElementInfo[]; // ordered by selection time
}
```

UX considerations:

• Need a way to deselect/remove a specific element (click the badge in sidepanel?)
• Visual feedback on the page: all selected elements should stay highlighted, not just the latest
• The highlight colors should be distinguishable (numbered or color-coded)
• Keep single-element as the simple default — multi-select could be a modifier key (Shift+click) or a toggle