feat(ext/shared): centralize error-message copy in ERROR_COPY map

Replaces the popup's regex-chain humanizeError with a total lookup over
every error code returned by extension/src/service-worker/router/. A
generated test discovers codes via grep so the registry can't drift.
The popup keeps its small set of regex translators for Rust/serde error
phrasing that doesn't go through the router's error vocabulary.

Subsumes B2 — fullscreen consumer lands in the next commit.

extension/src/shared/error-copy.ts

@@ -0,0 +1,102 @@
+export interface ErrorCta {
+  label: string;
+  action?: 'unlock' | 'reload_extension' | 'open_setup';
+}
+
+export interface ErrorCopy {
+  title: string;
+  body: string;
+  cta?: ErrorCta;
+}
+
+const UNLOCK_CTA: ErrorCta = { label: 'Unlock vault', action: 'unlock' };
+
+export const ERROR_COPY: Record<string, ErrorCopy> = {
+  vault_locked: {
+    title: 'Vault locked',
+    body: 'Unlock your vault to continue.',
+    cta: UNLOCK_CTA,
+  },
+  unauthorized_sender: {
+    title: 'Action not allowed',
+    body: 'This action is not allowed from here.',
+  },
+  unknown_message_type: {
+    title: 'Internal error',
+    body: 'The extension received an unknown request — try reloading.',
+    cta: { label: 'Reload extension', action: 'reload_extension' },
+  },
+  origin_mismatch: {
+    title: 'Wrong site',
+    body: 'This login belongs to a different site — refusing to leak credentials cross-origin.',
+  },
+  not_a_login: {
+    title: 'Not a login',
+    body: 'That item does not have a username and password to fill.',
+  },
+  no_totp: {
+    title: 'No 2FA on this item',
+    body: 'This item does not have a TOTP secret configured.',
+  },
+  invalid_sender_url: {
+    title: 'Cannot read tab URL',
+    body: 'The current tab has no recognizable URL — try reloading the page.',
+  },
+  tab_navigated: {
+    title: 'Tab changed',
+    body: 'The browser tab changed before the action could complete — try again.',
+  },
+  captured_tab_gone: {
+    title: 'Tab is gone',
+    body: 'The browser tab closed before the action could complete — try again.',
+  },
+  item_not_found: {
+    title: 'Item not found',
+    body: 'That item is no longer in the vault — it may have been deleted from another device.',
+  },
+  attachment_not_found: {
+    title: 'Attachment missing',
+    body: 'The attachment is referenced in the item but is not present in the vault.',
+  },
+  upload_failed: {
+    title: 'Upload failed',
+    body: 'Could not upload the attachment — check your connection and try again.',
+  },
+  download_failed: {
+    title: 'Download failed',
+    body: 'Could not download the attachment — check your connection and try again.',
+  },
+  'invalid base32 secret': {
+    title: 'Invalid secret',
+    body: 'The TOTP secret must be valid Base32 (letters A-Z and digits 2-7 only).',
+  },
+  'no items to import': {
+    title: 'Nothing to import',
+    body: 'The CSV did not contain any importable items.',
+  },
+  'no reference image stored locally': {
+    title: 'No reference image',
+    body: 'This device has no reference image saved locally — re-attach the device or restore from backup.',
+  },
+  'remote already contains a Relicario vault': {
+    title: 'Vault already exists',
+    body: 'The selected repository already contains a vault — use Attach existing instead of Create new.',
+  },
+  'Extension not configured. Run setup first.': {
+    title: 'Extension not configured',
+    body: 'Run setup before using this action.',
+    cta: { label: 'Open setup', action: 'open_setup' },
+  },
+  'Reference image not set. Run setup first.': {
+    title: 'Reference image missing',
+    body: 'Run setup to save your reference image.',
+    cta: { label: 'Open setup', action: 'open_setup' },
+  },
+};
+
+export function lookupErrorCopy(code: string): ErrorCopy {
+  return ERROR_COPY[code] ?? {
+    title: 'Something went wrong',
+    body: code,
+  };
+}