fix: propagate send failures, guard image conversion, cleanup speech mic (#130)

Seven correctness bugs in the session composer:

- sessions.ts sendMessage: await the prompt submission and rethrow on
  failure instead of a fire-and-forget .catch(), so handleSend's existing
  restore-draft-and-alert catch actually runs.
- pasteFromClipboard: route pasted images through toJpeg() so they get
  the same resize/compress treatment as picked/captured photos.
- pickFromLibrary/pickFromCamera: wrap toJpeg() in try/catch (and switch
  to Promise.allSettled for the multi-select batch) so one bad asset
  doesn't silently drop the whole batch; surface a new imageFailed alert.
- pickFromLibrary: cap selection at 10 images.
- useSpeech: abort the native recognition session on unmount so the mic
  doesn't stay hot after leaving the screen.
- Surface useSpeech's error via Alert, keyed on the error value so it
  fires once per distinct error.
- Undo on the revert banner now also clears the composer, since it was
  prefilled by the edit flow and could otherwise be sent as a duplicate.


Claude-Session: https://claude.ai/code/session_01T12AhSnQVrSxNnvwfCx2z6

Co-authored-by: engineer <engineer@macbookpro.lan>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/i18n/zh-Hans.json

@@ -118,7 +118,11 @@
       "replyFailedTitle": "回复失败",
       "replyFailedMessage": "无法发送您的回复。请重试。",
       "rejectFailedTitle": "拒绝失败",
-      "rejectFailedMessage": "无法发送您的回复。请重试。"
+      "rejectFailedMessage": "无法发送您的回复。请重试。",
+      "imageFailedTitle": "图片未添加",
+      "imageFailedMessage": "一张或多张图片无法处理。请尝试其他照片。",
+      "speechErrorTitle": "语音输入失败",
+      "speechErrorMessage": "无法使用语音输入。请检查麦克风权限后重试。"
     }
   },
   "connection": {