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/command/cmd.rs

@@ -2228,7 +2228,28 @@ effect_cmd!(
     "help",
     |_args: &[String]| -> Result<(), String> { Ok(()) },
     |_args: &Vec<String>, _ctx: &CmdContext| -> Vec<Box<dyn Effect>> {
-        vec![effect::change_mode(AppMode::Help)]
+        vec![
+            effect::help_page_set(0),
+            effect::change_mode(AppMode::Help),
+        ]
+    }
+);
+
+effect_cmd!(
+    HelpPageNextCmd,
+    "help-page-next",
+    |_args: &[String]| -> Result<(), String> { Ok(()) },
+    |_args: &Vec<String>, _ctx: &CmdContext| -> Vec<Box<dyn Effect>> {
+        vec![effect::help_page_next()]
+    }
+);
+
+effect_cmd!(
+    HelpPagePrevCmd,
+    "help-page-prev",
+    |_args: &[String]| -> Result<(), String> { Ok(()) },
+    |_args: &Vec<String>, _ctx: &CmdContext| -> Vec<Box<dyn Effect>> {
+        vec![effect::help_page_prev()]
     }
 );
 
@@ -2320,6 +2341,16 @@ pub fn default_registry() -> CmdRegistry {
     r.register_pure(&ExportCmd(vec![]), ExportCmd::parse);
     r.register_pure(&WriteCmd(vec![]), WriteCmd::parse);
     r.register_pure(&HelpCmd(vec![]), HelpCmd::parse);
+    r.register(
+        &HelpPageNextCmd(vec![]),
+        HelpPageNextCmd::parse,
+        |_args, _ctx| Ok(Box::new(HelpPageNextCmd(vec![]))),
+    );
+    r.register(
+        &HelpPagePrevCmd(vec![]),
+        HelpPagePrevCmd::parse,
+        |_args, _ctx| Ok(Box::new(HelpPagePrevCmd(vec![]))),
+    );
 
     // ── Navigation (unified Move) ──────────────────────────────────────
     r.register(