feat: show email preview snippet in thread list tiles

Added preview field to EmailThread (from latest email's preview via
_groupIntoThreads). Thread tiles now show subject + one-line body snippet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

next.md

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+# Next
+
+## Introduction
+
+Do one thing, ask if unsure first!
+
+Then implement.
+
+Then run `task check`.
+
+Then move task to done.md
+
+Check if all files are staged.
+
+Git repo should not contain unknown files.
+
+Then commit.
+
+## Tasks
+
+## Pull-to-refresh on email list
+
+`EmailListScreen` has a manual sync icon button but no swipe-to-refresh gesture.
+Wrap the `StreamBuilder` result body in a `RefreshIndicator` that calls the
+same sync trigger as the icon button.
+
+File: `lib/ui/screens/email_list_screen.dart`
+
+## Mark as unread button in email detail
+
+`EmailRepository.setFlag(emailId, seen: false)` already exists. Add a
+"Mark as unread" icon button (or overflow menu item) in `EmailDetailScreen`
+that calls `setFlag(seen: false)` then pops the screen so the message
+reappears as unread in the list.
+
+File: `lib/ui/screens/email_detail_screen.dart`
+
+## Quote original message in reply, and add Forward button
+
+When replying, `prefillBody` is never set so compose opens with an empty body.
+Set `prefillBody` to the original message formatted as a plain-text quote:
+
+```text
+\n\n— On <date>, <from> wrote:\n> line1\n> line2…
+```
+
+For Forward, add a third icon button (Icons.forward) next to reply/reply-all:
+
+- subject: `Fwd: <original subject>`
+- To/Cc: empty (user fills in)
+- body: same quoted original text
+
+The body snapshot is already available in `_buildBody()` context.
+
+File: `lib/ui/screens/email_detail_screen.dart`