terminal: fix posix_spawnp failure (spawn-helper exec bit), half-viewport default, fullscreen toggle, middle-click close

- Root cause of the endless 'disconnected, reconnecting' banner: pnpm
  strips the executable bit from node-pty's prebuilt spawn-helper, so every
  spawn failed with posix_spawnp failed and the client retried forever.
  ensureSpawnHelper() restores 0755 at plugin activation (mirrors
  dsh-pty-local's postinstall; works for link installs too). Verified
  against the live GUI: terminal now connects and echoes.
- TerminalView stops auto-reconnecting on a server refusal (close 1011 with
  a reason): shows the reason plus a manual retry instead of spinning.
- defaultShell() also guards an empty SHELL.
- New sessions open at half the viewport width; width clamping now allows
  filling the window, and a header fullscreen button expands the sidebar to
  the full viewport (toggle restores half).
- Middle-click on a tab closes it (auxclick button 1, autoscroll
  suppressed).
- Tests: +2 (shell fallback, spawn-helper exec bit idempotent).

src/client/sidebar.module.css

@@ -579,12 +579,31 @@
 
 .terminalBanner {
   flex: none;
+  display: flex;
+  align-items: center;
+  gap: 8px;
   padding: 3px 10px;
   font: var(--dsw-font-xxxs-11);
   color: var(--dsw-alias-state-warn-label);
   background: var(--dsw-alias-state-warn-tertiary);
 }
 
+.terminalRetry {
+  flex: none;
+  padding: 1px 8px;
+  border: 1px solid var(--dsw-alias-border-l2);
+  border-radius: 999px;
+  background: var(--dsw-alias-bg-layer-2);
+  color: var(--dsw-alias-label-secondary);
+  font: var(--dsw-font-xxxs-strong-11);
+  cursor: pointer;
+}
+
+.terminalRetry:hover {
+  background: var(--dsw-alias-interactive-bg-hover);
+  color: var(--dsw-alias-label-primary);
+}
+
 /* ── Git ───────────────────────────────────────────────────────────────── */
 
 .git {