Make the screen explain itself: labels, tools, help pages, hints

For developers new to submodules (notes/usability-plan.md, D32):
- x and X read "restore to pin" and "restore committed pins"
- problems only, filter and fetch are shown in the footer
- ? shows every key in titled columns, then a legend of the status
  words, symbols and spine roles; esc closes it
- u reads "pull (fast-forward)" on the superproject
- the detail pane suggests f when the last fetch is old
- a hint above the footer until the first key
- the header says "staged", like the tree

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/tui/keys_test.go

@@ -0,0 +1,16 @@
+package tui
+
+import "testing"
+
+func TestUpdateReadsPullOnTheSuperproject(t *testing.T) {
+	k := defaultKeys()
+	if got := k.forSelection(true).Update.Help().Desc; got != "pull (fast-forward)" {
+		t.Errorf("superproject: u reads %q", got)
+	}
+	if got := k.forSelection(false).Update.Help().Desc; got != "update pin" {
+		t.Errorf("submodule: u reads %q", got)
+	}
+	if got := k.Update.Help().Desc; got != "update pin" {
+		t.Errorf("forSelection changed the key map: u reads %q", got)
+	}
+}