fix(panel): home filter UI overlays instead of reflowing dashboard

[upsetbit]

Summary

Replaces the inline <details> disclosure on Home with a fixed-position right-side drawer (mirroring the session side panel). Opening filters no longer shifts the dashboard grid downward.

• New .dashboard-filters-toggle button + .dashboard-filters-drawer (fixed position, transform: translateX() transition, z-index above content).
• Backdrop click + ESC key close the drawer.
• URL-driven filter state and existing Alpine @change live-submit handler unchanged.
• Filter form layout switches to vertical inside the drawer for better usability.

Closes #226.

Test plan

• Run just build && ./bin/prosa-panel with PROSA_PANEL_DEV_LOGIN=1.
• Open / — click "Filters" → drawer slides in from the right, dashboard grid stays in place.
• Press ESC → drawer closes.
• Click the backdrop → drawer closes.
• Change a filter (window, agent, project, device) → form auto-submits (live), URL params persist on reload.
• With JavaScript disabled, drawer is fully visible (no x-cloak flash) and form still submits via the noscript Apply button.

🤖 Generated with Claude Code

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