feat(command): implement help page navigation

Implement help page navigation with `help-page-next` and `help-page-prev`
commands.

- Added `HelpPageNextCmd` and `HelpPagePrevCmd` to `src/command/cmd.rs` .
- Registered help navigation commands in `CmdRegistry` .
- Updated `HelpCmd` to initialize the help page.
- Added unit tests for help navigation in `src/ui/app.rs` .

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/ui/effect.rs

@@ -892,3 +892,42 @@ pub fn change_mode(mode: AppMode) -> Box<dyn Effect> {
 pub fn set_selected(row: usize, col: usize) -> Box<dyn Effect> {
     Box::new(SetSelected(row, col))
 }
+
+// ── Help page navigation ────────────────────────────────────────────────────
+
+#[derive(Debug)]
+pub struct HelpPageNext;
+impl Effect for HelpPageNext {
+    fn apply(&self, app: &mut App) {
+        let max = crate::ui::help::HELP_PAGE_COUNT.saturating_sub(1);
+        app.help_page = app.help_page.saturating_add(1).min(max);
+    }
+}
+
+#[derive(Debug)]
+pub struct HelpPagePrev;
+impl Effect for HelpPagePrev {
+    fn apply(&self, app: &mut App) {
+        app.help_page = app.help_page.saturating_sub(1);
+    }
+}
+
+#[derive(Debug)]
+pub struct HelpPageSet(pub usize);
+impl Effect for HelpPageSet {
+    fn apply(&self, app: &mut App) {
+        app.help_page = self.0;
+    }
+}
+
+pub fn help_page_next() -> Box<dyn Effect> {
+    Box::new(HelpPageNext)
+}
+
+pub fn help_page_prev() -> Box<dyn Effect> {
+    Box::new(HelpPagePrev)
+}
+
+pub fn help_page_set(page: usize) -> Box<dyn Effect> {
+    Box::new(HelpPageSet(page))
+}