Fixes #672 


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---

> [!NOTE]
> Add selectable problem rows with Select all/Clear all and Fix N
selected, and update tests to cover selection behavior.
> 
> - **UI (Problems panel)**:
> - Add checkbox selection for each problem row (`ProblemItem`) with row
click-to-toggle, `data-testid="problem-row"`, and accessibility
attributes.
> - Introduce selection state in `_Problems` with auto-select-all on
report load; provide Select all / Clear all controls.
> - Change Fix button to operate on selected problems only, showing
dynamic label `Fix N problem(s)` and disabled when none selected.
> - Wire `RecheckButton` to clear selection before rechecking; minor
hover style tweaks; add `Checkbox` component.
> - **E2E Tests**:
> - New test: selecting specific problems and fixing only selected; add
snapshots for prompt content.
> - Update manual edit tests (React/Vite, Next.js) to assert Fix button
enabled/disabled and counts; remove old ARIA snapshots.
>   - Minor import addition for `Timeout` and related expectations.
> 
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