fix(ui): preserve carousel keyboard and accessibility semantics

[kvnloo]

Follow-up to #441.

Credit to guidovizoso for the original bundled carousel diagnosis. I rechecked the relevant paths on current main before changing anything; three narrow problems were still present:

• the Embla reInit listener was registered but never removed;
• capture-phase Left/Right handling also intercepted editable descendants, and vertical carousels still used horizontal keys;
• the home “Explore agents” carousel exposed role="region" / aria-roledescription="carousel" without an accessible name.

Change

• pair both reInit / select subscriptions with cleanup;
• centralize keyboard ownership so horizontal uses Left/Right, vertical uses Up/Down, and inputs/textareas/selects/contenteditable keep native arrows;
• name the home carousel from its visible “Explore agents” heading.

Regression

Added a focused Bun/Happy DOM keyboard matrix for horizontal, vertical, and editable-target behavior.

Evidence boundary

This is source + deterministic test coverage. I have not claimed physical screen-reader/browser acceptance in this environment; that remains the final accessibility check before considering the slice complete.

AI assistance: an AI assistant rechecked current source against #441, prepared the narrow patch/test, and kept the original reporter’s provenance explicit.

Credit / provenance

• @guidovizoso reported and broke down the carousel keyboard/accessibility defects in The vendored carousel leaks a listener, steals arrow keys from inputs, and is unnamed to screen readers #441. This PR implements that diagnosed behavior.

[charan-rathore]

The three scoped fixes check out at 7a85d73: the capture handler uses the orientation-aware keyboard helper, editable targets keep their arrow keys, and cleanup removes both reInit and select using the registered callback. The home carousel's aria-labelledby points to its visible heading.

One new tooling issue remains: bunx biome check app/tests/carousel-keyboard.test.ts fails formatting on the long assertions. Please format that new test. The separate import/export-order finding in carousel.tsx also occurs on the original base file, so I am not attributing it to this change.

The committed tests exercise the helper, but not the actual capture handler or effect cleanup. Please consider adding component tests that render a vertical carousel and assert ArrowDown scrolls, then unmount and assert both event registrations are released with their original callbacks. I ran those as temporary review probes: the original carousel failed both, and this head passed. A labelled-region probe also passed.

Validation: helper matrix 3 passed; component probes 3 passed; home fallback-routing tests 2 passed; app/server/worker typechecks passed. These were isolated Bun 1.3.14 checks with a mocked Embla API, not a full database suite or browser/screen-reader audit.

For issue provenance, #441 was closed as filed in error. This review independently checks the three bugs scoped in this PR; it does not treat all six original issue items as reproduced or resolved.