feat: add persistent server-client sessions

   Make herdr persistent by default.

   Launching herdr now starts or reattaches to a background session
   server. Clients can detach and reattach while panes and agent
   processes keep running. Session mode now supports multi-client attach,
   auto-detect startup, and a thin-client/headless-server split.

   This also refactors the large app, ui, input, pane, config,
   workspace, and persistence modules into smaller focused submodules,
   while preserving behavior and colocating tests with the code they
   exercise.

   Upgrade notes:
   - persistence mode is now the default
   - in-app quit detaches the current client instead of stopping the server
   - use `herdr server stop` to stop the background session
   - use `--no-session` for the old single-process behavior
   - default socket paths now live under the config directory

1 files changed, 82 insertions, 0 deletions

diff --git a/src/app/ids.rs b/src/app/ids.rs
new file mode 100644
index 00000000..490e6051
--- /dev/null
+++ b/src/app/ids.rs
@@ -0,0 +1,82 @@
+use super::App;
+
+impl App {
+    pub(super) fn find_pane(
+        &self,
+        pane_id: crate::layout::PaneId,
+    ) -> Option<(usize, &crate::pane::PaneState)> {
+        self.state
+            .workspaces
+            .iter()
+            .enumerate()
+            .find_map(|(ws_idx, ws)| ws.pane_state(pane_id).map(|pane| (ws_idx, pane)))
+    }
+
+    pub(super) fn public_workspace_id(&self, ws_idx: usize) -> String {
+        self.state.workspaces[ws_idx].id.clone()
+    }
+
+    pub(super) fn public_tab_id(&self, ws_idx: usize, tab_idx: usize) -> Option<String> {
+        let ws = self.state.workspaces.get(ws_idx)?;
+        ws.tabs.get(tab_idx)?;
+        Some(format!("{}:{}", ws.id, tab_idx + 1))
+    }
+
+    pub(super) fn public_pane_id(
+        &self,
+        ws_idx: usize,
+        pane_id: crate::layout::PaneId,
+    ) -> Option<String> {
+        let ws = self.state.workspaces.get(ws_idx)?;
+        let pane_number = ws.public_pane_number(pane_id)?;
+        Some(format!("{}-{pane_number}", ws.id))
+    }
+
+    pub(super) fn parse_workspace_id(&self, id: &str) -> Option<usize> {
+        self.state
+            .workspaces
+            .iter()
+            .position(|workspace| workspace.id == id)
+            .or_else(|| id.strip_prefix("w_")?.parse::<usize>().ok()?.checked_sub(1))
+            .or_else(|| id.parse::<usize>().ok()?.checked_sub(1))
+    }
+
+    pub(super) fn parse_tab_id(&self, id: &str) -> Option<(usize, usize)> {
+        if let Some(rest) = id.strip_prefix("t_") {
+            let (ws_raw, tab_raw) = rest.rsplit_once('_')?;
+            let ws_idx = self.parse_workspace_id(ws_raw)?;
+            let tab_idx = tab_raw.parse::<usize>().ok()?.checked_sub(1)?;
+            self.state.workspaces.get(ws_idx)?.tabs.get(tab_idx)?;
+            return Some((ws_idx, tab_idx));
+        }
+
+        let (ws_raw, tab_raw) = id.rsplit_once(':')?;
+        let ws_idx = self.parse_workspace_id(ws_raw)?;
+        let tab_idx = tab_raw.parse::<usize>().ok()?.checked_sub(1)?;
+        self.state.workspaces.get(ws_idx)?.tabs.get(tab_idx)?;
+        Some((ws_idx, tab_idx))
+    }
+
+    pub(super) fn parse_pane_id(&self, id: &str) -> Option<(usize, crate::layout::PaneId)> {
+        if let Some(rest) = id.strip_prefix("p_") {
+            if let Some((ws_raw, pane_raw)) = rest.rsplit_once('_') {
+                let ws_idx = self.parse_workspace_id(ws_raw)?;
+                let pane_id = crate::layout::PaneId::from_raw(pane_raw.parse::<u32>().ok()?);
+                return Some((ws_idx, pane_id));
+            }
+
+            let pane_id = crate::layout::PaneId::from_raw(rest.parse::<u32>().ok()?);
+            return self.find_pane(pane_id).map(|(ws_idx, _)| (ws_idx, pane_id));
+        }
+
+        let (ws_raw, pane_number_raw) = id.rsplit_once('-')?;
+        let ws_idx = self.parse_workspace_id(ws_raw)?;
+        let pane_number = pane_number_raw.parse::<usize>().ok()?;
+        let ws = self.state.workspaces.get(ws_idx)?;
+        let pane_id = ws
+            .public_pane_numbers
+            .iter()
+            .find_map(|(pane_id, number)| (*number == pane_number).then_some(*pane_id))?;
+        Some((ws_idx, pane_id))
+    }
+}