fix: detach expanded search panel from header width constraints (min-width, viewport-anchored)

## Problem

The expanded `.panel` in `ol-search-bar` is currently bound to the width of its parent `.search-outer` via `position: absolute; left: 0; right: 0`. As the header shrinks at narrower viewports, the panel shrinks with it, causing two visible breakdowns:

- **Below ~945px** — facets in `.pf-bar` start to overflow the panel width
- **Below ~795px** — the barcode scanner icon overflows the search box and the layout visually breaks

This also means there's no guaranteed minimum usable width for the expanded experience on desktop/tablet.

## How GitHub and Hardcover.app solve this

Both apps **detach** the expanded search popup from the header's width. The visual trick:
- The popup is anchored to the **right edge** of the triggering search input (not the left)
- It has a fixed/minimum width independent of the header's shrinking width
- The search input *inside* the popup aligns vertically with the header input — so it appears to be the same element expanding in place
- GitHub: ~100% viewport width up to 1024px, then clamps to ~600px; right edge aligned with the search bar

## Proposed fix (surgical — no redesign)

Change `.panel` from parent-width-constrained absolute positioning to a **viewport-anchored fixed position** computed in JS when the panel opens:

1. On expand, read the bounding rect of the input element
2. Set `position: fixed; top: ; right: ` on the panel
3. Set `min-width: 600px` (or similar) so it never collapses below a usable width on desktop
4. On resize/scroll, recompute (or close the panel)
5. **Mobile full-screen overlay (<600px) stays completely intact** — no changes there
6. All internals (facets, suggestions, chips, autocomplete) are untouched

This is the same "detached popup" pattern used by GitHub and Hardcover.app.

## Acceptance criteria

- [ ] Expanded panel never goes below ~600px wide on desktop/tablet
- [ ] Facets never overflow at any desktop viewport width
- [ ] Right edge of expanded panel aligns with right edge of the search input
- [ ] No regression on mobile full-screen overlay (<600px)
- [ ] No regression on embedded (search-page) mode
- [ ] Panel repositions or closes on window resize

## Files likely touched

- `frontend/src/components/ol-search-bar.js` — panel positioning logic + CSS
- Tests: `ol-search-bar.mobile-overlay.test.js`, potentially new desktop breakpoint tests

Start in frontend/src/components/ol-search-bar.js by reading the existing panel positioning and breakpoint logic. Run ol-search-bar.mobile-overlay.test.js, then add or run desktop breakpoint coverage for the listed acceptance criteria, including embedded mode and resize behavior. Done means the desktop/tablet panel stays usable and aligned while the mobile overlay remains unchanged.

Written by the indexing model from the issue text.