feat(ask): resizable, persisted thread sidebar (Part A of #8)

[mvanhorn]

Summary

Implements Part A of #8: a resizable, persisted thread sidebar
on /ask. The fixed w-60 (240px) is now a drag-resizable column
between 180px and 480px, persisted in useUiPrefsStore so the
chosen width survives reloads.

Changes

src/web/stores/ui-prefs-store.ts (~30 lines)

• New askSidebarWidth: number field (default 240).
• Exported ASK_SIDEBAR_MIN_WIDTH=180, ASK_SIDEBAR_MAX_WIDTH=480,
  ASK_SIDEBAR_DEFAULT_WIDTH=240 so the page can render aria-valuemin
  / max without re-deriving them.
• New setAskSidebarWidth(width) setter clamps to the bounds and
  persists; load() re-clamps on read so a stale localStorage value
  can never push the sidebar off-screen or shrink it below the
  "New conversation" button's minimum legible width.

src/web/pages/AskPage.tsx (~80 lines)

• Sidebar <aside> drops w-60 and uses inline style={{ width }}
  driven by the store value; flex-shrink-0 is preserved.
• New 1px-wide drag handle (with 4px hover hitbox) at the sidebar's
  right edge, rendered only at md+ breakpoints (matching the sidebar
  itself).
• During drag the mouse listeners attach to document so a fast
  movement that leaves the handle's hitbox keeps tracking; global
  cursor: col-resize and userSelect: none are applied for the
  drag duration so the browser doesn't paint text selection across
  the layout.
• Keyboard accessibility: Left/Right arrow keys step by 16px, Home /
  End jump to the clamp endpoints. ARIA role="separator" with
  aria-valuemin / valuemax / valuenow reflect the live width.

Acceptance criteria from #8

• Sidebar resizes smoothly, width persists across reloads.
  Width is read from localStorage on store init, clamped on
  both read and set, and rewritten on every drag end.

Part B (shiki-based syntax highlighting in MarkdownContent.tsx)
is left for a follow-up PR — it adds a new dependency and the
"no runtime flash via async load" requirement is substantial enough
to review separately. Keeping Part A independently shippable means
the sidebar improvement can land without waiting on the highlighter
work.

Verification

bun install
bun run typecheck   # clean

Manual checks I ran in bun run dev:

• Drag the handle: sidebar grows / shrinks smoothly to the cursor
  position. Stops at 180px (handle disappears under the "New
  conversation" button if you keep dragging left) and 480px.
• Reload the page: width is restored.
• Tab to the handle, press Right several times: width grows in 16px
  steps; ARIA aria-valuenow updates accordingly.
• Tab to the handle, press End: width snaps to 480; Home: 180.
• Resize while text is selected in the conversation column: drag
  starts cleanly without losing the selection mid-drag (userSelect
  toggle).

The pre-existing biome warnings on lines 137-141 / 245-265 of
AskPage.tsx are not touched by this PR — keeping the diff scoped to
the sidebar change.