feat(desktop): rebindable keyboard shortcuts panel

Add a central keybind registry + nanostore so desktop hotkeys are
discoverable and user-rebindable. A titlebar ⌨ button (and ⌘/) opens a
collapsible map grouped by Composer (read-only) / Profiles / Session /
Navigation / View; click any chip to capture a new combo. Overrides
persist to localStorage as a delta against shipped defaults, so future
default changes aren't shadowed by a stored snapshot.

Migrates the previously scattered inline listeners (palette, command
center, new session, sidebar, theme) into the registry, and adds profile
switch/cycle/create + default-profile hotkeys.

apps/desktop/src/styles.css

@@ -1188,3 +1188,42 @@ canvas {
     animation: none;
   }
 }
+
+/* ── Keybind panel / edit overlay: small key chips ──────────────────────────
+   A quiet `kbd`-style chip shared by the shortcuts panel and the on-screen
+   editor so both read as the same control. No animation, no glow. */
+.kbd-cap {
+  display: inline-grid;
+  place-items: center;
+  min-width: 1.5rem;
+  height: 1.4rem;
+  padding: 0 0.4rem;
+  border-radius: 0.375rem;
+  font-family: var(--dt-font-mono, ui-monospace, monospace);
+  font-size: 0.72rem;
+  font-weight: 500;
+  line-height: 1;
+  color: color-mix(in srgb, var(--dt-foreground) 82%, transparent);
+  background: color-mix(in srgb, var(--ui-bg-elevated) 70%, transparent);
+  border: 1px solid var(--ui-stroke-secondary);
+  box-shadow: inset 0 -1px 0 color-mix(in srgb, var(--ui-stroke-tertiary) 50%, transparent);
+}
+
+/* Unbound slot: a hollow dashed chip inviting a binding. */
+.kbd-cap--ghost {
+  color: color-mix(in srgb, var(--dt-foreground) 42%, transparent);
+  background: none;
+  border-style: dashed;
+  border-color: var(--ui-stroke-tertiary);
+  box-shadow: none;
+  font-style: italic;
+}
+
+/* Waiting for a keypress: solid accent, no motion. */
+.kbd-capturing {
+  color: var(--theme-primary);
+  border-color: color-mix(in srgb, var(--theme-primary) 55%, var(--ui-stroke-secondary)) !important;
+  border-style: solid;
+  background: color-mix(in srgb, var(--theme-primary) 9%, var(--ui-bg-elevated));
+  box-shadow: none;
+}