feat(frontend): add batch pause/resume + confirmation modal to StreamsTable bulk actions (#762)

[utosjr1913]

Summary

Closes #762.

Bulk selection (checkboxes, select all/deselect all, sequential batch cancel) was
already implemented in StreamsTable.tsx. This PR adds the three pieces the issue
still required:

1. Pause and Resume batch actions, alongside the existing Cancel
2. A confirmation modal showing the affected count, gating all three actions
   behind an explicit confirm step
3. Per-stream failure reporting in the UI — previously failures only went to
   console.error

Changes

• New: frontend/src/components/BulkActionModal.tsx — confirm → running →
  complete modal, reusing the app's existing .modal-backdrop/.modal-panel
  styles and useFocusTrap hook (same pattern as the existing
  ClaimBatchModal.tsx).
• frontend/src/components/StreamsTable.tsx:
  • isStreamSelectable now includes paused streams (needed so Resume has
    eligible targets)
  • BulkActionBar now renders Pause / Resume / Cancel, all routed through the
    new confirmation modal instead of firing immediately
  • Generalized the old cancel-only handler into requestBulkAction /
    runBulkAction, which collects { streamId, message } failures per stream
• frontend/src/index.css: added the .bulk-action-bar styles, which were
  referenced in the component but never defined anywhere — the floating action
  bar has been rendering completely unstyled until this PR.
• Updated BULK_SELECTION_VISUAL_GUIDE.md and BULK_SELECTION_IMPLEMENTATION.md
  to match.

Verification against acceptance criteria

• Checkbox column, select all/deselect all — pre-existing.
• Batch actions: cancel, pause, resume — Pause/Resume added.
• Confirmation modal showing count.
• Batch cancel triggers individual cancel per stream — generalized to
  pause/resume too.
• Partial failures shown per stream — listed by stream ID in the modal.
• UI matches BULK_SELECTION_VISUAL_GUIDE.md — guide updated, missing CSS added.

Known blocker for CI verification

frontend/package.json is empty in this repo (pre-existing, unrelated) —
npm install fails immediately, so I couldn't run the test suite locally.
Flagging for a separate fix.

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