feat(ui): app-wide keyboard and screen-reader accessibility

[Tryanks]

Accessibility pass built on the pinned GPUI's AccessKit support (roles, aria props, tab_index, focus_visible, automatic Enter/Space activation — verified in vendored source).

• Shared accessible_clickable treatment in material.rs: tab stop + semantic role + localized accessible name + theme-aware 2px focus outline; applied to all raw clickable surfaces (composer send/steer/stop, chat Git/Open actions, sidebar search/settings/project/thread rows, terminal tabs, diff tabs/rows, settings nav/popovers, toast disclosures, recent-project rows).
• Hover-hidden row actions (new/archive) now also reveal on keyboard focus.
• Composite widgets (palette, composer suggestion/model/approval menus): ListBox/Option semantics with aria_selected + active-descendant tracking while focus stays in the input.
• Settings toggle rows own the Switch role + keyboard activation (upstream Switch is mouse-only at rev 0315556 — rechecked — and is now visual-only inside the row).
• 22 localized screen-reader names added to both locales; role/focus conventions documented in DESIGN.md.

Verification: fmt / clippy -D warnings / workspace tests green (139 UI tests incl. new regression tests for focus order, Enter/Space activation, palette navigation, toggle inversion). Manual walkthrough in the built app: full Tab/Shift-Tab traversal with no dead ends, real activations (thread switch, settings toggle, composer menu, palette) and native AccessKit tree inspection showing localized names/roles. VoiceOver was not enabled — no auditory claim made. Known remainder: some pre-existing icon-only component buttons are focusable but unnamed; keyboard resizing/scrollbars/terminal-grid SR output deliberately out of scope.

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