fix(sidebar): make the sidebar resize handle keyboard-operable

[pedrofrxncx]

The sidebar's resize handle (SidebarResizeHandle) is role="separator" but had no tabIndex and no onKeyDown — a keyboard-only user had no way to resize the sidebar at all, only drag it with a mouse. This is a real accessibility gap: WAI-ARIA's separator pattern for a moveable divider expects it to be focusable and support arrow-key adjustment plus aria-valuenow/valuemin/valuemax, none of which were present.

Changes:

• use-sidebar-resize.ts: exposes minWidth, maxWidth, and a new adjustWidth(delta) setter (reusing the existing clamp/localStorage persistence), plus a small SIDEBAR_RESIZE_KEYBOARD_STEP constant.
• sidebar-resize-handle.tsx: adds tabIndex={0}, a focus ring, aria-valuenow/min/max, and an onKeyDown handler — ArrowLeft/ArrowRight step the width, Home/End jump to min/max, Enter resets to default (mirrors the existing double-click reset).
• org-shell-layout/index.tsx: wires the new props through from useSidebarResize().

No behavior change for existing mouse/touch drag-resize or double-click-reset; this only adds a previously-missing keyboard path.

To confirm: focus the resize handle (desktop, expanded sidebar) with Tab and use the arrow keys — the sidebar width should change; Home/End should snap to min/max; Enter should reset to the default width.

Verified locally: bun run fmt, cd apps/web && bunx tsc --noEmit (one pre-existing unrelated mustache module error confirmed present on main via git stash, not touched by this change), bunx oxlint on all three changed files (0 warnings/errors). No test added — this is a pure UI interaction/accessibility change with no trust boundary; full CI covers the rest.

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Summary by cubic

Make the sidebar resize handle keyboard-operable to meet the WAI-ARIA separator pattern and close an accessibility gap. Before: the handle was not focusable and only worked with mouse drag. Now: it is focusable and supports Arrow/Home/End keys and Enter to reset; mouse/touch drag and double-click reset are unchanged.

Review notes

• SidebarResizeHandle: adds tabIndex={0}, aria-valuenow/min/max, a focus ring, and onKeyDown (ArrowLeft/Right ±16px, Home/End to min/max, Enter resets).
• useSidebarResize: exposes minWidth, maxWidth, and adjustWidth(delta); clamping and persistence are unchanged; exports SIDEBAR_RESIZE_KEYBOARD_STEP=16.
• OrgShellLayout: wires the new props to the handle.
• Migration: if you render SidebarResizeHandle elsewhere, pass width, minWidth, maxWidth, onAdjustWidth, and optionally onResetWidth.

^{Written for commit 17f8f5a. Summary will update on new commits.}