feat: optionally sync clipboard between connected sessions (#15934)

* feat(clipboard): optionally sync clipboard between connected sessions

Clipboard content received from a remote session is written to the local
clipboard with an owner marker, so the client clipboard loop deliberately
skips re-broadcasting it to avoid echo loops. As a result, text copied in
one remote window could not be pasted in another connected remote window.

Add an opt-in local option (allow-sync-clipboard-between-sessions) that
relays Clipboard/MultiClipboards messages received from one session to
all other connected sessions, excluding the source session. Per-session
clipboard permissions and view-only mode are still respected via the
existing send path, and the owner marker on the receiving peers prevents
any echo back.

Desktop (flutter) only; file clipboard is not affected.

* fix(lang): propagate sync-clipboard-between-sessions-tip to all locale files

Add the new key to template.rs and every locale file per the localization
convention, move the en.rs entry to the end of the list, and drop comments
that only restated the names next to them.

* fix(lang): add the 'Sync clipboard between sessions' label to the localization catalog

The checkbox label goes through translate(), so add it to template.rs
and every locale file so non-English locales can translate it. en.rs is
skipped since the English display text is identical to the key.

* fix(clipboard): check the source session's full clipboard permission before relaying

The relay was gated only by the incoming clipboard_allowed check
(!disable_clipboard && !view_only). Gate it with
is_text_clipboard_required() instead, which additionally respects the
source session's server_clipboard_enabled and server_keyboard_enabled
state, matching the predicate already applied to destination sessions.
A message arriving after the source permission was revoked (or from a
non-conforming peer) is no longer propagated to other sessions. The
existing local update_clipboard behavior is unchanged.

* fix(lang): translate the new clipboard sync entries in all locale files

Fill the 'Sync clipboard between sessions' label and its tooltip in
every locale file instead of leaving them blank, following each file's
existing terminology. template.rs keeps the empty master entries.

src/lang/tw.rs

@@ -761,5 +761,7 @@ pub static ref T: std::collections::HashMap<&'static str, &'static str> =
         ("Continue", "繼續"),
         ("Browser didn't open? Use the url below to sign in.", "瀏覽器未開啟？請使用下方網址登入。"),
         ("Lock canvas", "鎖定畫布"),
+        ("Sync clipboard between sessions", "在工作階段間同步剪貼簿"),
+        ("sync-clipboard-between-sessions-tip", "在一個遠端工作階段中複製的文字或圖片也會傳送到其他已連線工作階段的剪貼簿。"),
     ].iter().cloned().collect();
 }