feat: add page scrolling, open-row, and tab-advance

Add new navigation and editing capabilities to improve data entry and
browsing efficiency.

- Implement `PageScroll` command for PageUp/PageDown navigation.
- Implement `OpenRecordRow` command (Vim-style 'o') to add a row and enter edit
  mode.
- Implement `CommitAndAdvanceRight` command for Excel-style Tab navigation.
- Add keybindings for Home, End, PageUp, PageDown, and Tab.
- Update UI hints to reflect new Tab functionality.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)

src/ui/app.rs

@@ -253,7 +253,7 @@ impl App {
     pub fn hint_text(&self) -> &'static str {
         match &self.mode {
             AppMode::Normal => "hjkl:nav  i:edit  R:records  P:prune  F/C/V:panels  T:tiles  [:]:page  >:drill  ::cmd",
-            AppMode::Editing { .. } => "Enter:commit  Esc:cancel",
+            AppMode::Editing { .. } => "Enter:commit  Tab:commit+right  Esc:cancel",
             AppMode::FormulaPanel => "n:new  d:delete  jk:nav  Esc:back",
             AppMode::FormulaEdit { .. } => "Enter:save  Esc:cancel  — type: Name = expression",
             AppMode::CategoryPanel => "jk:nav  Space:cycle-axis  n:new-cat  a:add-items  d:delete  Esc:back",