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/pages/client/sidebar/SettingsTab.tsx

@@ -1,4 +1,5 @@
-import React, { useState } from 'react';
+import React, { useEffect, useState } from 'react';
+import { useAtom } from 'jotai';
 import {
   Badge,
   Box,
@@ -23,7 +24,8 @@ import { useMatrixClient } from '../../../hooks/useMatrixClient';
 import { getMxIdLocalPart, mxcUrlToHttp } from '../../../utils/matrix';
 import { nameInitials } from '../../../utils/common';
 import { useMediaAuthentication } from '../../../hooks/useMediaAuthentication';
-import { Settings } from '../../../features/settings';
+import { Settings, SettingsPages } from '../../../features/settings';
+import { settingsRequestAtom } from '../../../state/settingsRequest';
 import { useUserProfile } from '../../../hooks/useUserProfile';
 import { Modal500 } from '../../../components/Modal500';
 import { useSetting } from '../../../state/hooks/settings';
@@ -172,6 +174,21 @@ export function SettingsTab() {
   const profile = useUserProfile(userId);
 
   const [settingsOpen, setSettingsOpen] = useState(false);
+  // [Gitea #159] Programmatic "open settings at page X" requests.
+  const [settingsRequest, setSettingsRequest] = useAtom(settingsRequestAtom);
+  const [initialPage, setInitialPage] = useState<SettingsPages | undefined>();
+  useEffect(() => {
+    if (!settingsRequest) return;
+    const page = {
+      general: SettingsPages.GeneralPage,
+      account: SettingsPages.AccountPage,
+      notifications: SettingsPages.NotificationPage,
+      devices: SettingsPages.DevicesPage,
+    }[settingsRequest];
+    setInitialPage(page);
+    setSettingsOpen(true);
+    setSettingsRequest(null);
+  }, [settingsRequest, setSettingsRequest]);
 
   const displayName = profile.displayName ?? getMxIdLocalPart(userId) ?? userId;
   const avatarUrl = profile.avatarUrl
@@ -201,7 +218,7 @@ export function SettingsTab() {
       <PresencePicker />
       {settingsOpen && (
         <Modal500 requestClose={() => setSettingsOpen(false)}>
-          <Settings requestClose={() => setSettingsOpen(false)} />
+          <Settings initialPage={initialPage} requestClose={() => setSettingsOpen(false)} />
         </Modal500>
       )}
     </SidebarItem>