fix(web-ui): SplitPane a11y, logic bugs, and test coverage (#518)

* fix(web-ui): SplitPane a11y, logic bugs, and test coverage

- Remove aria-hidden from divider; add role=separator + aria-orientation
- Add type="button" to collapse buttons (prevent form submission)
- Add keyboard resize via ArrowLeft/ArrowRight (WCAG 2.1.1)
- Fix lastNonCollapsed corruption when cross-collapsing panes
- Don't write 0/100 to localStorage on collapse (prevents reload drift)
- Clamp readStorage to valid expanded range [min, 100-min]
- Return null until isMobile resolves (prevent hydration flash)
- Expand tests: 29 tests covering all 8 review findings

* fix(web-ui): address CodeRabbit review on SplitPane

- Extract commitExpandedSplit helper shared by drag and keyboard paths
- Fix aria-valuemin/aria-valuemax to reflect actual collapsed state range

* fix(web-ui): guard keyboard resize at collapsed sentinels (0/100)

* fix(web-ui): SplitPane CodeRabbit round 3 — clamp, click guard, ARIA

- readStorage: clamp fallback and stored values into [min,max]; reject
  collapsed sentinels (0/100) explicitly via Number.isFinite path
- onDividerMouseDown: reset livePercent to splitPct and dragMoved=false
- onMouseUp: only call commitExpandedSplit when dragMoved — plain clicks
  on divider no longer reopen collapsed panes
- ARIA range: use splitPct===0/===100 (not <=/>= minPanePercent)
- Tests: stale-width clamp (10→15, 90→85), invalid defaultSplit clamp,
  plain-click no-op on collapsed pane (37 tests total)

* Update web-ui/src/components/sessions/SplitPane.tsx

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

* fix(web-ui): guard outward drags from collapsed edges as no-ops

Moves dragged from a fully-collapsed edge (splitPct 0 or 100) that keep
the pointer at or beyond the same edge no longer set dragMoved or update
pane widths; only inward movement that crosses back inside the container
exits the collapsed state and triggers commitExpandedSplit.

Adds four regression tests covering both left- and right-collapsed
outward and inward drag paths.

* fix(web-ui): hoist collapse buttons out of role=separator (WAI-ARIA 1.2)

A focusable separator is a widget and must not contain interactive
descendants. Wraps the resize handle and both collapse buttons in a
sibling-level track div; role="separator" now has no children so AT
reaches all three focusable elements independently.

Visual output is unchanged — buttons remain absolutely positioned over
the same pixel area. Updates the mobile-hidden test to target the track
instead of the separator.

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Co-authored-by: Test User <test@example.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: 3 people