fix keyboard navigation

[t1]

• shift-tab from anywhere within the content jumps to the operation tab; this should only happen from the first focusable in the content
• tab on the response schema status tabs focusses the next tab, but it should select the next focusable instead

[t1]

Agent: starting work

Type: Bug fix
Assessment: Two keyboard navigation issues:

1. Shift+Tab from anywhere in content jumps to operation tab (lines 1066-1072 in app.js) - should only happen from the first focusable element
2. Status code tabs activate on focus (lines 1139-1143) which steals Tab navigation - should only Tab to next focusable instead

Approach:

1. Fix Shift+Tab handler to check if we're at the first focusable in content before jumping to tabs
2. Remove or modify focusin handler for status tabs and use Tab/arrow keys instead
   Files: core/src/main/resources/com/github/t1/openapi/ui/generator/app.js, add browser tests in BrowserTest.java

[t1]

Agent: done

Commit: b0be351
Summary: Fixed two keyboard navigation issues:

1. Shift-Tab from anywhere in content now only jumps to the operation tab when pressed from the FIRST focusable element. From other elements, it uses the browser's default reverse tab order.
2. Tab on response schema status tabs now moves focus to the next focusable element without activating the tab. Arrow keys still activate status tabs as expected.

Tests:

• Added shouldNotJumpToTabOnShiftTabFromSecondField to verify Shift-Tab from non-first elements doesn't jump to tab
• Added shouldFocusNextElementOnTabFromStatusCodeTab to verify Tab on status tabs doesn't activate them
• All 361 core tests pass

CI: Not pushed (local testing only)