feat(frontend): swap Toast off Radix onto Tamagui (#581)

[gaidheal1]

Summary

Second sub-issue of #578 to ship, stacked on #580's PR (#701) since it uses the Tamagui toolchain that PR lands. Closes #581. Base is claude/issue-580-progressbar-tamagui, not development — merge #701 first.

Replaces @radix-ui/react-toast in ToastContext.tsx and ToastManager.tsx with Tamagui's Toast/ToastProvider/ToastViewport.

Confirmed before writing any code that Tamagui's toast primitives mirror Radix's API and internal announce-region technique closely (same duration/type/swipeDirection prop names, same "visible element + transient hidden announce copy" live-region pattern) — by reading @tamagui/toast's source, not by assumption. This made the swap close to mechanical: ToastContext's public shape (useToast().showToast) and ToastManager's props (messages, onDismiss) are unchanged; no call site moves.

Two real behavioral differences found (by diffing actual rendered DOM, not inferred from docs)

• role="status" placement differs. Radix's visible toast is a bare <li> with no ARIA role (only its transient announce copy carries role="status"); Tamagui's fork puts role="status" on both the visible element (aria-live="off") and the announce copy (aria-live="polite" for type="background") — a different shape, same announcement politeness. Updated the test to use getAllByRole accordingly and assert the aria-live value directly rather than assume a single match.
• List semantics are lost — recorded as an accepted a11y gap, not silently dropped. Radix's Toast.Root/Viewport render as real <li>/<ol> (native list navigation for screen readers across stacked toasts). Tamagui's are View-based (styled(YStack, ...)) with no tag-override that actually takes effect from this call site — tried tag="li"/tag="ol" first, verified empirically they're silently ignored (Tamagui's compiled output stayed div-based). Documented in ToastManager.tsx and the test file, per Remove Radix dependencies and verify accessibility parity across all replaced components #587's "any accepted regression recorded explicitly, not silently absorbed" baseline.

New test coverage

Added two tests for an acceptance-criterion behavior no prior test — even under Radix — exercised: auto-dismiss firing after the configured duration, and the timer not firing (or throwing) after an early unmount. Uses fake timers against a short per-test ToastProvider duration.

Styling

unstyled set on both Toast and ToastViewport so Toast.module.scss stays the sole source of visual truth, same pattern #580 established for ProgressBar. data-state="open"/"closed" (which the SCSS's fadeInUp/fadeOut keyframes key off) carries over unchanged.

Verification

• 449/449 unit tests pass
• tsc --noEmit clean
• npm run lint: unchanged from the 14 pre-existing problems
• Production build succeeds. Bundle delta on top of Replace Radix Progress in ProgressBar (or delete the component) #580's baseline: +17.6 kB raw / +6.5 kB gzip on the index chunk (measured directly) — modest, since Replace Radix Progress in ProgressBar (or delete the component) #580 already paid the fixed Tamagui-provider cost.

Not verified: npm run test:a11y — needs a live dev server + Django backend via Docker, unavailable in this sandbox. Same category of disclosed gap as #580's unverified Storybook a11y suite, not a claim of a passing run.

Acceptance criteria (from #581)

• No @radix-ui/react-toast import remains in either file
• useToast().showToast(...) still works from every existing call site with no signature change
• Toasts auto-dismiss after the configured duration (default 3300ms) and clean up their timers on unmount — newly tested
• Multiple simultaneous toasts stack in the same order as today
• Screen-reader announcement verified — politeness level equivalent to Radix's type="background", checked via explicit live-region assertion
• toastsFeature flag still gates rendering (unchanged, not touched by this swap)
• ToastManager.test.tsx passes (updated, no longer imports Radix)
• npm run test:a11y — not verified, see above
• Confirmed Tamagui's Toast can be driven by external open/onOpenChange state without requiring a Trigger in the tree — matches Radix's model directly, no ref workaround needed

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