feat(react): first-class notification severity

[soufian3hm]

Closes #36.

Frontend slice of first-class notification severity.

What

• @chimely/client: WellKnownPayload gains an optional severity: 'high' | 'medium' | 'low' -- additive, in keeping with its "only ever gains optional fields" contract.
• @chimely/react: the default item renders a left accent bar tinted by three new appearance variables -- colorSeverityHigh (#dc2626), colorSeverityMedium (#d97706), colorSeverityLow (#2563eb). The accent is decorative (aria-hidden); the value already rides the item text.
• Payloads pass through Chimely verbatim, so any value outside the union renders no accent.
• Tabs can already filter on severity via their filter predicate, per the issue.

Deferred (as the issue frames it)

• Bell tinting by highest unread severity and severity-filterable server counts need the server-side counts work first (Server-side filtered counts for tabs #39), so they are out of this slice.
• No server change -- payloads pass through verbatim.

Verification

biome, pnpm --filter "./packages/*" build, typecheck, and vitest are all green: @chimely/client 54 tests and @chimely/react 108 tests, including 3 new ones (known-severity accent, no accent without severity, and an out-of-set value rendering nothing). A minor changeset covers both packages.

[greptile-apps]

Greptile Summary

This PR adds first-class notification severity to the frontend packages. The main changes are:

• Adds optional severity to the client well-known payload type.
• Adds React appearance variables for severity accent colors.
• Renders a decorative accent bar for known values in the default item.
• Adds tests for known, missing, and out-of-set values.
• Adds a changeset for both frontend packages.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The renderer keeps payload values intact for filters and custom renderers.
• The new style variables line up with the existing appearance forwarding path.

Important Files Changed

Filename	Overview
packages/client/src/types.ts	Adds an optional well-known payload field while preserving the existing custom payload extension shape.
packages/react/src/appearance.ts	Adds optional appearance variables that match the CSS custom-property names used by the new accent styles.
packages/react/src/components/DefaultItem.tsx	Adds a local guard for known values and renders the decorative accent without mutating the original payload.
packages/react/src/styles.ts	Adds positioning and color styles for the accent bar with safe CSS fallbacks.
packages/react/src/inbox.test.tsx	Adds coverage for known, absent, and unsupported payload values.
.changeset/notification-severity.md	Adds a minor release note for the client and React packages.

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