refactor(contacts): Implement multi-page Contact Management with advanced sorting

Split Contact Management into 3 dedicated pages for improved mobile usability:
- /contacts/manage - Settings & navigation hub (manual approval + cleanup)
- /contacts/pending - Full-screen pending contacts view
- /contacts/existing - Full-screen existing contacts with search/filter/sort

New Features:
- Advanced sorting: Name (A-Z/Z-A) & Last advert (newest/oldest)
- URL-based sort state (?sort=name&order=asc)
- Activity indicators: 🟢 active, 🟡 recent, 🔴 inactive
- Changed terminology: "Last seen" → "Last advert" (more accurate)
- Cleanup tool moved from Settings modal to Contact Management page

Technical Changes:
- Created contacts_base.html standalone template
- Split contacts.html into 3 specialized templates
- Refactored contacts.js for multi-page support with page detection
- Added 2 new Flask routes: /contacts/pending, /contacts/existing
- Removed cleanup section from base.html Settings modal

Mobile-first design: Each page has full-screen space with touch-friendly
navigation and back buttons.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/templates/base.html

@@ -244,19 +244,6 @@
                     <button type="button" class="btn-close" data-bs-dismiss="modal"></button>
                 </div>
                 <div class="modal-body">
-                    <h6>Contact Management</h6>
-                    <hr>
-                    <div class="mb-3">
-                        <label for="inactiveHours" class="form-label">Remove contacts inactive for:</label>
-                        <div class="input-group">
-                            <input type="number" class="form-control" id="inactiveHours" value="48" min="1">
-                            <span class="input-group-text">hours</span>
-                        </div>
-                    </div>
-                    <button class="btn btn-danger" id="cleanupBtn">
-                        <i class="bi bi-trash"></i> Clean Inactive Contacts
-                    </button>
-                    <hr>
                     <h6>Device Information</h6>
                     <div id="deviceInfo" class="small text-muted">
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