fix(web): code review round 3 -- UI bugs and remaining design fidelity

- FileAttachment never surfaced a failed authenticated download -- the
  card looked identically clickable whether it worked or not, and
  clicking while broken silently no-op'd. Added an error state/retry
  (useAuthedBlobUrl gained a retry() escape hatch for this).
  ImageAttachment/AudioAttachment already handled this; only the file
  card didn't.
- --shadow-btn was one token serving two different specs: round 2's fix
  (0 6px 16px, from the 50px auth CTAs) actually regressed the 36px
  messaging send button (ConversationView/VoiceRecorder), which needs
  0 4px 12px per 02-messages.dc.html. Split into --shadow-btn /
  --shadow-btn-sm.
- loadOlder's scroll-restore guard compared topicId by value, which
  false-positives on an A->B->A round trip within one request's
  lifetime (same string, but the DOM was torn down and rebuilt by the
  intervening open/close). Switched to a generation counter, same
  pattern as topics.ts's leave-race fix.
- errorToast wasn't included in ConversationView/DetailColumn's
  topicId-reset effects -- a failed action's toast could keep showing
  over a conversation/contact switched to afterward. Also switched
  those effects (and MessagesLayout's) from useEffect to
  useLayoutEffect -- since none of these components remount on
  topicId change, a post-paint effect could let one frame render with
  the previous topicId's stale UI still showing.
- VoiceRecorder never listened for MediaRecorder's own onerror -- a
  mid-recording failure left the "录音中" timer ticking forever with no
  indication the recording had already died.
- VerificationCodeButton's phone-change countdown reset (round 2) only
  cleared the button's own state; RegisterPage/ResetPasswordPage's
  challengeId was still tied to whatever phone the code was actually
  issued for, with nothing cross-checking it against a since-edited
  phone number.
- Agreement's checkbox used bare accent-color on a native input, which
  only tints the checked fill and leaves size/radius/border to the
  browser/OS; replaced with the spec's exact 16x16 border-radius:4px
  box + check glyph.
- Added a catch-all route (App.tsx), TenantFeedbackPage image-picker
  validation feedback + corrected remove-button style, and a handful of
  smaller token/spacing corrections against screens/*.dc.html (NavRail
  font-weight, LoginPage/TenantEntryPage logo size+shadow+color,
  ChatListColumn row padding/icon size, --shadow-modal alpha).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/index.css

@@ -103,10 +103,14 @@
 
   /* Shadows */
   --shadow-card: 0 8px 32px rgba(35, 74, 125, 0.10);
-  --shadow-modal: 0 20px 60px rgba(0, 0, 0, 0.18);
+  --shadow-modal: 0 20px 60px rgba(0, 0, 0, 0.2);
   --shadow-menu: 0 12px 36px rgba(0, 0, 0, 0.16);
   --shadow-bubble: 0 1px 2px rgba(0, 0, 0, 0.06);
+  /* screens/01-account.dc.html's 50px auth CTAs (登录/注册/提交...). */
   --shadow-btn: 0 6px 16px rgba(54, 139, 250, 0.2);
+  /* screens/02-messages.dc.html's 36px 发送 button (ConversationView/VoiceRecorder) -- a smaller
+     button gets a smaller shadow, not the same token as the 50px auth buttons above. */
+  --shadow-btn-sm: 0 4px 12px rgba(54, 139, 250, 0.2);
   --shadow-focus-ring: 0 0 0 3px rgba(22, 119, 255, 0.1);
 }