feat(e2ee): undecryptable placeholder says why and offers the fix (#159)

'Unable to decrypt message' now carries one sentence per matrix-js-sdk
DecryptionFailureCode (describeDecryptionFailure, unit-tested against every
code so no raw code can leak into the copy) and, where something fixes it,
one button: no key backup → 'Set up key backup'; backup exists but this
session can't open it / key withheld for an unverified session → 'Unlock key
backup' / 'Verify this session' (both open Settings → Devices via a new
settingsRequestAtom that SettingsTab consumes); backup working or unknown
session (rust-crypto re-requests keys itself) → 'Retry', which re-runs
decryptEventIfNeeded. Sender-side problems are plain text. The raw code sits
in the placeholder's tooltip for support.

Verified headless on a fresh session in the encrypted seed room: each event
shows 'Sent before you signed in here, and no key backup exists…' with
tooltip HISTORICAL_MESSAGE_NO_KEY_BACKUP; the button opens Settings → Devices.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/decryptionReason.test.ts

@@ -0,0 +1,44 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { DecryptionFailureCode } from 'matrix-js-sdk/lib/crypto-api';
+import { describeDecryptionFailure } from './decryptionReason';
+
+test('every SDK failure code maps to a sentence without the raw code in it', () => {
+  Object.values(DecryptionFailureCode).forEach((code) => {
+    const r = describeDecryptionFailure(code);
+    assert.ok(r.text.length > 10, code);
+    assert.ok(!r.text.includes(code), `raw code leaked for ${code}`);
+    assert.ok(!/[A-Z_]{8,}/.test(r.text), `shouty code-like text for ${code}: ${r.text}`);
+    if (r.action !== 'none') assert.ok(r.actionLabel, `action without label for ${code}`);
+  });
+});
+
+test('the fixable cases carry the right action', () => {
+  assert.equal(
+    describeDecryptionFailure(DecryptionFailureCode.HISTORICAL_MESSAGE_NO_KEY_BACKUP).action,
+    'setup-backup',
+  );
+  assert.equal(
+    describeDecryptionFailure(DecryptionFailureCode.HISTORICAL_MESSAGE_BACKUP_UNCONFIGURED).action,
+    'verify-session',
+  );
+  assert.equal(
+    describeDecryptionFailure(DecryptionFailureCode.HISTORICAL_MESSAGE_WORKING_BACKUP).action,
+    'retry',
+  );
+  assert.equal(
+    describeDecryptionFailure(DecryptionFailureCode.MEGOLM_KEY_WITHHELD_FOR_UNVERIFIED_DEVICE)
+      .action,
+    'verify-session',
+  );
+  assert.equal(
+    describeDecryptionFailure(DecryptionFailureCode.MEGOLM_UNKNOWN_INBOUND_SESSION_ID).action,
+    'retry',
+  );
+  assert.equal(
+    describeDecryptionFailure(DecryptionFailureCode.UNKNOWN_SENDER_DEVICE).action,
+    'none',
+  );
+  assert.equal(describeDecryptionFailure(undefined).action, 'none');
+  assert.equal(describeDecryptionFailure('SOMETHING_NEW').text, 'Unable to decrypt this message.');
+});

src/app/utils/decryptionReason.ts

@@ -0,0 +1,75 @@
+import { DecryptionFailureCode } from 'matrix-js-sdk/lib/crypto-api';
+
+/**
+ * [Gitea #159] What to tell the user about an undecryptable message, and the
+ * one thing that fixes it (if anything does). Raw codes never reach the UI
+ * text — they go in the tooltip for support.
+ */
+export type DecryptionAction = 'setup-backup' | 'retry' | 'verify-session' | 'none';
+
+export type DecryptionReason = {
+  text: string;
+  action: DecryptionAction;
+  actionLabel?: string;
+};
+
+export const describeDecryptionFailure = (
+  code: DecryptionFailureCode | string | null | undefined,
+): DecryptionReason => {
+  switch (code) {
+    case DecryptionFailureCode.HISTORICAL_MESSAGE_NO_KEY_BACKUP:
+      // No backup exists on the server at all: this key is gone for this
+      // session (another signed-in device may still share it), and the fix is
+      // for the future.
+      return {
+        text: 'Sent before you signed in here, and no key backup exists — set one up so your next session can read history.',
+        action: 'setup-backup',
+        actionLabel: 'Set up key backup',
+      };
+    case DecryptionFailureCode.HISTORICAL_MESSAGE_BACKUP_UNCONFIGURED:
+      // A backup exists but this session can't open it (needs the recovery key
+      // / verification).
+      return {
+        text: "Sent before you signed in here. Your key backup exists but this session can't open it yet.",
+        action: 'verify-session',
+        actionLabel: 'Unlock key backup',
+      };
+    case DecryptionFailureCode.HISTORICAL_MESSAGE_WORKING_BACKUP:
+      return {
+        text: 'Waiting for the key from your backup…',
+        action: 'retry',
+        actionLabel: 'Retry',
+      };
+    case DecryptionFailureCode.HISTORICAL_MESSAGE_USER_NOT_JOINED:
+      return { text: 'Sent before you joined this room.', action: 'none' };
+    case DecryptionFailureCode.MEGOLM_KEY_WITHHELD_FOR_UNVERIFIED_DEVICE:
+      return {
+        text: "The sender's device won't share keys with unverified sessions.",
+        action: 'verify-session',
+        actionLabel: 'Verify this session',
+      };
+    case DecryptionFailureCode.MEGOLM_KEY_WITHHELD:
+      return { text: 'The sender chose not to share the key for this message.', action: 'none' };
+    case DecryptionFailureCode.MEGOLM_UNKNOWN_INBOUND_SESSION_ID:
+    case DecryptionFailureCode.OLM_UNKNOWN_MESSAGE_INDEX:
+      // rust-crypto re-requests the key from our other devices by itself
+      // (there is no public re-request API any more); a retry re-runs the
+      // decryption in case it has arrived since.
+      return {
+        text: "This session doesn't have the key yet — it is being requested from your other devices.",
+        action: 'retry',
+        actionLabel: 'Retry',
+      };
+    case DecryptionFailureCode.SENDER_IDENTITY_PREVIOUSLY_VERIFIED:
+      return {
+        text: "The sender's identity changed since you verified them — re-verify to read this.",
+        action: 'none',
+      };
+    case DecryptionFailureCode.UNSIGNED_SENDER_DEVICE:
+      return { text: "Sent from a device the sender hasn't verified.", action: 'none' };
+    case DecryptionFailureCode.UNKNOWN_SENDER_DEVICE:
+      return { text: "Sent from a device we don't know about.", action: 'none' };
+    default:
+      return { text: 'Unable to decrypt this message.', action: 'none' };
+  }
+};