feat(ui): label github_hmac as 'GitHub / Sentry (X-Hub-Signature-256)' in signing mode picker

[marvin-tensorleap]

Why this change exists

When TEN-321 was investigated, github_hmac already correctly handled GitHub's X-Hub-Signature-256 format — but three GitHub webhook triggers were created with hmac_sha256 instead. The root cause: the signing mode dropdown showed raw internal keys (github_hmac, hmac_sha256) with no labels, making it impossible to guess which one to pick for a GitHub webhook.

This is the second fix from TEN-325: surface github_hmac as the obviously-correct choice before the next misconfiguration occurs.

How this PR fixes it

• Adds a signingModeLabels map that gives each mode a human-readable name. github_hmac is now displayed as "GitHub / Sentry (X-Hub-Signature-256)" — the header name makes its purpose unambiguous.
• Both the new-trigger creation form and the existing trigger editor now use labels instead of raw keys.
• The existing trigger editor now also shows the description hint below the select (it previously only appeared in the creation form), so users editing an existing trigger can see what the selected mode does.

Technical summary

• ui/src/pages/RoutineDetail.tsx: add signingModeLabels constant; use signingModeLabels[mode] ?? mode in both SelectItem maps; add description <p> to TriggerEditor's webhook section.

Verification

Change is purely display-layer. The underlying signingMode value written to the API is unchanged (still "github_hmac", "hmac_sha256", etc.) — the label only affects what the user reads in the dropdown. TypeScript pattern is identical to the existing signingModeDescriptions map.