fix: unit tests for deriveEvents, type=button, aria-label, aria-pressed (#433 #438 #439 #440)

[observerr411]

Summary

• closes good first issue: add unit tests for deriveEvents() in hooks/use-activity-feed.ts #433 — Added unit tests for deriveEvents() in __tests__/hooks/use-activity-feed.test.ts: covers all four event types (created, withdrawal, cancelled, completed), verifies role (sent/received) for each, and asserts the correct description text.
• closes good first issue: add missing type="button" to streams page filter/clear buttons #440 — Added type="button" to the token filter buttons, status filter buttons, and clear-filters button in app/app/streams/page.tsx to prevent accidental form submission.
• closes good first issue: give the dashboard's stale-price warning emoji accessible text #439 — Replaced title="Price may be outdated" with role="img" aria-label="Price may be outdated" on the stale-price warning emoji in components/streams/dashboard-stats.tsx for reliable screen-reader announcement.
• closes good first issue: add aria-pressed to create-form USD/token amount-mode toggle #438 — Added aria-pressed={!usdInputMode} / aria-pressed={usdInputMode} to the token-amount / USD-amount toggle buttons in app/app/create/create-form.tsx so screen readers can announce the active mode.

Test plan

• Run npm test — new deriveEvents tests should all pass
• Inspect streams page in browser: token/status/clear filter buttons no longer lack type
• Use a screen reader on the dashboard with a stale price: emoji is announced as "Price may be outdated"
• Use a screen reader on the create form: toggling amount mode announces aria-pressed state changes

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