Consolidate web errorMessage onto the shared helper

[selfcontained]

What

Completes the errorMessage regression sweep started server-side in #914. apps/web had its own copy of the helper plus one inline ternary; both now use the canonical apps/server/src/shared/lib/error-message.ts.

• Deleted the byte-identical reimplementation errorMessage in apps/web/src/components/app/jobs-helpers.tsx and re-pointed its four importers (jobs-add-dialog, jobs-detail-pane, jobs-prompt-tab, jobs-settings-tab).
• Replaced the inline err instanceof Error ? err.message : String(err) in apps/web/src/components/ui/markdown.tsx:211 (mermaid render error path).
• Addedapps/web/src/lib/errors.ts, a thin re-export of the shared helper.
• Dropped the errorMessage describe block from jobs-helpers.test.tsx — apps/server/test/error-message.test.ts already covers a strict superset (adds Error subclasses and null).

Net −19 lines. Zero behavior change: both removed expressions were byte-equivalent to the helper body.

Why this shape

Web imports a value here, not a type, so per the #844/#849 pattern it needs a dependency-free source module — shared/lib/error-message.ts qualifies (zero imports). The new @/lib/errors re-export keeps web call sites on @/lib/... instead of reaching across the apps/server boundary directly, matching lib/agent-types.ts, lib/ide-types.ts, and lib/media-upload.ts. finalize:web confirms the value import bundles cleanly.

Deliberately NOT swept

A grep for instanceof Error across apps/web/src returns 29 non-test sites. Only the 2 above were byte-equivalent; the other 27 carry distinct fallbacks and must keep them. Enumerated so a future run doesn't re-litigate:

• Custom fallback strings (22): "Failed to load jobs.", "Failed to save.", "Failed to send test.", "Failed to save browser notifications.", "Failed to load/save IDE settings.", "Failed to create agent.", "Couldn't load branches.", "Failed to save cross-repo messaging setting.", "Upload failed", "Failed to load personalities.", "Failed to set active personality.", "Failed to delete.", "Failed to save prompt delivery setting.", "Failed to load/save agent type settings.", "Failed to check for updates", "Failed to start assisted update", "Session connection failed." — plus "Failed to load release info" / `Failed to promote ${tag}`.
• undefined fallback (2): session-settings-dialog.tsx:61, agent-card-header.tsx:80 — a toast description that is deliberately omitted for non-Errors.
• null fallback (1): persona-launcher.tsx:183.
• Message is transformed, not just extracted (4): cleanError(err.message) in release-admin.tsx ×2 and use-release-updates.ts ×2; `Couldn't start update: ${err.message}` in release-available-toast.tsx:154.

Validation

pnpm run check, pnpm run finalize:web, web vitest (732 passed / 56 files), pnpm run test:e2e (178 passed, 12 skipped) — all green. Server unit tests not run: no backend files changed.

Next run

Queued: formatDate name collision — three identically-named one-line wrappers over @/lib/format with three different behaviors (activity-chart-utils.ts:3, release-utils.ts:24, jobs-helpers.ts:43).

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