Add an errorMessage(e: unknown) helper and retire 101 'as Error' casts

[InfinityBowman]

77 catch blocks in non-test source do (err as Error).message (101 counting tests). If a non-Error is thrown, the message is undefined and the user sees an empty error.

// packages/web/src/stores/authStore.ts -- 10 occurrences in this file alone
catch (err) { set({ authError: (err as Error).message }); }

useUnknownInCatchVariables is already on via strict, so err is correctly unknown -- the cast is throwing that away.

Fix

One helper in @corates/shared:

export function errorMessage(e: unknown): string {
  return e instanceof Error ? e.message : String(e);
}

Name it to avoid getErrorMessage(code: string), which already exists in packages/shared/src/errors/helpers.ts and maps an error code to its default message. Then convert the call sites. Note that several files already do this correctly inline (err instanceof Error ? err.message : String(err) appears in the guards), so the helper also de-duplicates that.

Done when

• Helper exists in @corates/shared
• No as Error in non-test source (currently 77)

Effort: 2-3 hours, mechanical.

Carried over from packages/docs/audits/typescript-audit-2026-04.md item 9, which measured ~100 in April counting tests. It is 101 now on the same basis, 77 in non-test source.

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Part of #778 (TypeScript correctness target state and tracker).