Context bar: match panes by live cwd via OSC 7 (was keyed on unset leaf.cwd)

The context indicator never showed because it matched on leaf.cwd, which
is almost always undefined (newLeaf sets none; the shell picker never
supplies one) — so the cwd<->transcript match never hit.

Fix: report each WSL pane's real working directory.
- pty.rs: inject PROMPT_COMMAND (forwarded via WSLENV) so the WSL shell
  emits OSC 7 (file://host/path) on every prompt. Default Ubuntu bash
  inherits an env-provided PROMPT_COMMAND; a shell that hard-assigns it,
  or a non-bash login shell, just won't report (indicator stays hidden,
  no breakage).
- XtermPane: register an OSC 7 handler, decode the path, emit onCwd.
- LeafPane: track liveCwd from onCwd and match the session on
  (liveCwd ?? leaf.cwd). OSC 7 fires at the bash prompt right before
  'claude' launches, so liveCwd is exactly claude's launch cwd; it also
  follows 'cd'.

tsc clean. Rust builds on the Windows host; needs runtime verification.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src-tauri/src/pty.rs

@@ -354,6 +354,23 @@ fn build_command(spec: &SpawnSpec) -> Result<(CommandBuilder, &'static str)> {
             let resolved_cwd = cwd.as_deref().unwrap_or("~");
             c.arg("--cd");
             c.arg(resolved_cwd);
+            // Make the shell report its working directory via OSC 7 on every
+            // prompt, so the frontend can map this pane to the claude session
+            // running in it (the context-fill indicator; see usage.rs +
+            // LeafPane). We set PROMPT_COMMAND in the environment and forward it
+            // through WSLENV — default Ubuntu bash inherits an env-provided
+            // PROMPT_COMMAND. A user shell that hard-assigns PROMPT_COMMAND (or
+            // a non-bash login shell) simply won't report, and the indicator
+            // stays hidden for that pane — no breakage either way.
+            c.env(
+                "PROMPT_COMMAND",
+                r#"printf '\033]7;file://%s%s\033\\' "$HOSTNAME" "$PWD""#,
+            );
+            let wslenv = match std::env::var("WSLENV") {
+                Ok(v) if !v.is_empty() => format!("{v}:PROMPT_COMMAND/u"),
+                _ => "PROMPT_COMMAND/u".to_string(),
+            };
+            c.env("WSLENV", wslenv);
             Ok((c, "failed to spawn wsl.exe; is WSL installed?"))
         }
         SpawnSpec::Powershell => {