fix(chat): unify context rail hover states, dropdown surfaces, and radii

[kennylauren]

Category: fix
User Impact: Hovering cards and menus in the chat context rail is now visible in light mode, dropdowns match the rail's surface color and corner radius in dark mode, and non-clickable rows no longer light up on hover.

Problem: The rail's hover fills used sidebar-accent, which is near-invisible in light mode (gray-50 on white) while fine in dark; its dropdowns inherited the overflow-menu popover surface, which reads too dark against the rail's card family in dark mode; dropdown radii didn't relate to the rail container's; the Changes tab stacked three nested surfaces; and disabled branch rows still showed hover feedback.

Solution: Move all rail hover/selected fills to muted — identical pixels in dark (where muted ≡ the old sidebar-accent value) and visible in light — matching the design system's own RowButton menu convention. Rail dropdowns now use bg-card with the rail's rounded-md, and their rows use rounded-sm concentric inside the padding. The Changes tab drops its filled slab and hover-pill overlay in favor of flat rounded-sm rows with the standard fills, and hovers are scoped with enabled: so disabled rows stay inert while the active row keeps its aria-current highlight.

Note for design reviewers: sq agents review --local flags two deliberate deviations from docs/color-token-mapping.md: dropdown surfaces here use card (not popover) and menu highlights use muted (not accent). Both were chosen intentionally — the dark popover token is reserved-darker for overflow menus (BOT-1600) and doesn't suit rail-attached dropdowns, and light accent on white surfaces is the invisible-hover defect this PR fixes; RowButton already uses muted for these states. If we keep this, the mapping doc could use a follow-up alignment.

Before / After (Light, Default)

Before	After
before-default-light.mov	after-default-light.mov

Before / After (Light, Multi-workspace)

Before	After
before-multi-light.mov	after-multi-light.mov

Before / After (Dark, Default)

Before	After
before-default-dark.mov	after-default-dark.mov

Before / After (Dark, Multi-workspace)

Before	After
before-multi-dark.mov	after-multi-dark.mov

File changes

src/features/chat/ui/widgets/WorkingContextPicker.tsx
Legacy (single-workspace) worktree/branch pickers: hover fills to muted, dropdown surfaces to bg-card rounded-md p-1.5 with concentric rounded-sm rows, search field highlight made hover-only, and hover scoped to enabled rows so the current/checked-out-elsewhere branches stay inert.

src/features/chat/ui/widgets/WorkspaceContextPicker.tsx
Multi-workspace pickers: same treatment as the legacy pickers, plus the trigger cards' resting fill moves from background/45 (darker than the rail surface in dark mode) to muted/60 to match the legacy cards, and the active row gains the aria-current selected fill.

src/features/chat/ui/widgets/ChangesWidget.tsx
De-nests the Changes tab: removes the filled scroll slab and its colored bottom fade plus the per-row hover-pill overlay, replacing them with flat rounded-sm rows using the standard muted hover and a shared scrolling list that fades scrolled rows under a top mask like the picker dropdowns.

src/features/chat/ui/widgets/WorkspaceActionsMenu.tsx
Legacy rail actions menu: content surface to bg-card rounded-md, items to rounded-sm with muted hover/focus/highlight fills, dropping the no-op sidebar-foreground text swaps.

src/features/chat/ui/widgets/WorkspaceRowActionsMenu.tsx
Multi-workspace row actions menu: content surface to bg-card rounded-md (items keep rounded-xs, concentric inside the px-3 gutter), highlight fills to muted.

src/features/chat/ui/widgets/WorkspaceWidget.tsx
Multi-workspace widget rows (add-workspace, change-folder, section actions menu): hover fills to muted, section menu surface to bg-card rounded-md p-1.5.

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