feat: Capsule dashboard filtering, sorting & pagination

[phertyameen]

PR Description

What changed

server/routes/capsules.js - rewrote the capsule list endpoint to accept query parameters: status (repeatable), search (iLike on title + message), unlockAfter/unlockBefore, sortBy, sortDir, page, and limit. All inputs are whitelist-validated before hitting the DB. Response now includes a meta object with total, totalPages, hasNextPage, and hasPrevPage.
app/dashboard/page.jsx - replaced the flat list with a filterable, sortable, paginated table. Search is debounced at 350ms with AbortController cancellation on rapid input. Status toggles, date range pickers, and a sort selector all reset to page 1 automatically. Pagination renders a windowed page list (first, last, ±2 around current) with ellipsis gaps.

Acceptance criteria met

• Sender can filter by draft, sealed, and unlocked — combinable
• Search and sort hit real API data; no client-side filtering
• Pagination handles seeded demo datasets cleanly at default limit of 20

Closes #375
Closes #74
Closes #373
Closes #372

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