fix(settings): redesign About Updates as hero card with check animation

[quiet-node]

Summary

Refactors the Updates section in Settings → About into a centered hero card matching the macOS Software Update aesthetic. The "Check for updates" action now plays the same green circle-draw + checkmark animation already used by the "Unload now" button in the AI tab, giving Settings a consistent feedback language for async actions.

Why

The previous layout duplicated the version (already shown immediately under the Thuki mascot in the About header) as a "Current version" row, surfaced a bare "Last checked" timestamp without any status context, and offered a small "Check now" button at the row's right edge. The result felt disconnected: three pieces of information stacked without answering the user's first question — am I up to date?

Changes

• Replaces the two-row UPDATES table with a single centered hero card.
• Hero shows a green check mark with "Thuki is up to date" in the idle state, or an amber pulse with "Thuki vX.Y.Z is ready" when an update is pending.
• "Last checked X minutes ago" now sits as a quiet caption below the status, eliminating the "Last checked" label noise.
• Primary "Check for updates" button below the caption. While a check is in flight, the button disables and renders the green draw-circle plus checkmark animation, matching the existing pattern in ModelTab for the "Unload now" action.
• Drops the redundant "Current version" row entirely.
• Adds data-state attributes on the hero so the available-update state gets a subtle amber tint, mirroring the existing settings banner accent.

Test plan

• bun run test:all:coverage clean (1232 frontend + 753 Rust tests, 100% line and function coverage)
• bun run validate-build clean (no signing key required locally; CI handles signing on release)
• Manual: idle state renders "Thuki is up to date" with green check icon and quiet last-checked caption
• Manual: clicking the button disables it, plays the circle + checkmark animation for 1.1 seconds, then reverts
• Manual: rapid double-click only fires one check_for_update invocation (button disabled state guards against re-entry)
• Manual: when an update is detected, hero switches to amber pulse + "Thuki X.Y.Z is ready" copy