fix(media): named search input + grid-only fragment for media selector

[PlusA2M]

Description

The "Select Media" picker's search was broken two ways: the input had no name
(so hx-include="[name='search']" sent nothing), and the endpoint always returned
the full panel into the inner grid (so each keystroke nested a fresh search box +
grid). This makes search filter in place without nesting.

Fixes #890

Changes

• admin-media.ts GET /selector: add name="search" to the input so the term
  is actually sent.
• Return a grid-only fragment when the request targets the grid
  (HX-Target: media-selector-grid), and the full panel only on the initial
  modal load — so keystrokes update the grid in place instead of nesting a panel.
• Build the card markup once and reuse it for both responses; move the empty state
  inside the grid (col-span-full).
• Also re-query on the search input's native clear (hx-trigger … , search).
• Add a regression test: initial load returns a named input + grid; an HTMX search
  returns only cards (no <input>, no grid container) with the term bound; empty
  search renders the in-grid empty state.

Testing

Unit Tests

• Added/updated unit tests (admin-media-selector.test.ts)
• All unit tests passing (npm test — full suite green)

E2E Tests

• Added/updated E2E tests
• All E2E tests passing

Checklist

• Code follows project conventions
• Tests added/updated and passing
• Type checking passes
• No console errors or warnings
• Documentation updated (if needed) — n/a