Badge.tsx (938 lines) has zero unit tests — controlled-input invariants untested

[Loule95450]

Context

The Badge is the user's primary interaction surface on the web: it detects the field, attaches a shadow-DOM panel, generates the password, writes it to the input, handles save/rotate banners. It is 938 lines and has zero unit tests; only detect.ts (the field-finder) is unit-tested.

Problem / Observation

• extension/src/content/Badge.tsx (938 lines): writeInput, showSaveBanner, showRotateBanner, attachBadge, Badge component lifecycle, UnlockForm, ReadyView, focus/key handlers.
• tests/detect.test.ts exists and is solid.
• tests/e2e/content.spec.ts covers a few end-to-end smokes via Playwright but doesn't drive the panel through every state machine transition.
• No vitest test of writeInput's React-controlled-component invariants (calling the prototype setter, firing both input and change events) — a regression here breaks every controlled-input login form on the web silently.
• No test of the findHost traversal that walks shadow boundaries in click-outside handling.
• No test of the rotate banner's heuristic for which input is current vs. new (hint matching on autocomplete, name, id containing "current" or "old").

Proposed approach

Add tests/badge.test.ts (or split into tests/badge-*.test.ts) with happy-dom fixtures. Specifically:

• writeInput calls the prototype setter AND fires input + change events (verify with spies).
• showSaveBanner removes any previous banner before adding a new one (DOM assertion).
• The rotate banner's "current vs new" split: given two password inputs, one with name="currentPassword" and one with id="newPassword", the old value is written to the first and the new value to the second.
• findHost returns the host element when given a node nested 3 shadow roots deep (mirrors what happens with iframe + shadow root sites).

Acceptance criteria

• At least one new test file with 8+ assertions covering Badge logic.
• Coverage report shows src/content/Badge.tsx above 50% line coverage (currently ~0%).
• CI is green.