client-core: hoist transcript_fold::{fold_page,raw_seq} out of desktop-app

Both desktop-app's app.rs and the new Android transcript client (RUST.md's
I5) need the same page-fold and live-stream resume-cursor logic; per
CODE_RULES's "write the logic once" it now lives in client-core alongside
fold_event/group_tool_runs instead of being duplicated. desktop-app calls
the shared functions; its own copies and their tests moved with them.

Also fixes a clippy::collapsible_if in config.rs's percent_decode, found
while re-running clippy after this change (let-chains are stable now).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client-core/src/config.rs

@@ -78,14 +78,14 @@ fn percent_decode(s: &str) -> String {
     let mut out = Vec::with_capacity(bytes.len());
     let mut i = 0;
     while i < bytes.len() {
-        if bytes[i] == b'%' && i + 2 < bytes.len() {
-            if let Ok(byte) =
+        if bytes[i] == b'%'
+            && i + 2 < bytes.len()
+            && let Ok(byte) =
                 u8::from_str_radix(std::str::from_utf8(&bytes[i + 1..i + 3]).unwrap_or(""), 16)
-            {
-                out.push(byte);
-                i += 3;
-                continue;
-            }
+        {
+            out.push(byte);
+            i += 3;
+            continue;
         }
         out.push(bytes[i]);
         i += 1;