Implement six major improvements to typing tutor

1. Start in Adaptive Drill by default: App launches directly into a
   typing lesson instead of the menu screen.

2. Fix error tracking for backspaced corrections: Add typo_flags
   HashSet to LessonState that persists error positions through
   backspace. Errors at a position are counted even if corrected,
   matching keybr.com behavior. Multiple errors at the same position
   count as one.

3. Fix keyboard visualization with depressed keys: Enable crossterm
   keyboard enhancement flags for key Release events. Track depressed
   keys in a HashSet with 150ms fallback clearing. Depressed keys
   render with bright/bold styling at highest priority. Add compact
   keyboard mode for medium-width terminals.

4. Responsive UI for small terminals: Add LayoutTier enum (Wide >=100,
   Medium 60-99, Narrow <60 cols). Medium hides sidebar and shows
   compact stats header and compact keyboard. Narrow hides keyboard
   and progress bar entirely. Short terminals (<20 rows) also hide
   keyboard/progress.

5. Delete sessions from history: Add j/k row navigation in history
   tab, x/Delete to initiate deletion with y/n confirmation dialog.
   Full chronological replay rebuilds key_stats, letter_unlock,
   profile scoring, and streak tracking. Only adaptive sessions update
   key_stats/letter_unlock during rebuild. LessonResult now persists
   lesson_mode for correct replay gating.

6. Improved statistics display: Bordered summary table on dashboard,
   WPM bar graph using block characters (green above goal, red below),
   accuracy Braille trend chart, bordered history table with WPM goal
   indicators and selected-row highlighting, character speed
   distribution with time labels, keyboard accuracy heatmap with
   percentage text per key, worst accuracy keys panel, new 7-month
   activity calendar heatmap widget with theme-derived intensity
   colors, side-by-side panel layout for terminals >170 cols wide.

Also: ignore KeyEventKind::Repeat for typing input, clamp history
selection to visible 20-row range, and suppress dead_code warnings
on now-unused WpmChart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/session/lesson.rs

@@ -1,3 +1,4 @@
+use std::collections::HashSet;
 use std::time::Instant;
 
 use crate::session::input::CharStatus;
@@ -8,6 +9,7 @@ pub struct LessonState {
     pub cursor: usize,
     pub started_at: Option<Instant>,
     pub finished_at: Option<Instant>,
+    pub typo_flags: HashSet<usize>,
 }
 
 impl LessonState {
@@ -18,6 +20,7 @@ impl LessonState {
             cursor: 0,
             started_at: None,
             finished_at: None,
+            typo_flags: HashSet::new(),
         }
     }
 
@@ -40,13 +43,6 @@ impl LessonState {
             .count()
     }
 
-    pub fn incorrect_count(&self) -> usize {
-        self.input
-            .iter()
-            .filter(|s| matches!(s, CharStatus::Incorrect(_)))
-            .count()
-    }
-
     pub fn wpm(&self) -> f64 {
         let elapsed = self.elapsed_secs();
         if elapsed < 0.1 {
@@ -56,12 +52,20 @@ impl LessonState {
         (chars / 5.0) / (elapsed / 60.0)
     }
 
+    pub fn typo_count(&self) -> usize {
+        self.typo_flags.len()
+    }
+
     pub fn accuracy(&self) -> f64 {
-        let total = self.input.len();
-        if total == 0 {
+        if self.cursor == 0 {
             return 100.0;
         }
-        (self.correct_count() as f64 / total as f64) * 100.0
+        let typos_before_cursor = self
+            .typo_flags
+            .iter()
+            .filter(|&&pos| pos < self.cursor)
+            .count();
+        ((self.cursor - typos_before_cursor) as f64 / self.cursor as f64 * 100.0).clamp(0.0, 100.0)
     }
 
     pub fn cpm(&self) -> f64 {
@@ -83,6 +87,7 @@ impl LessonState {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::session::input;
 
     #[test]
     fn test_new_lesson() {
@@ -105,4 +110,52 @@ mod tests {
         assert!(lesson.is_complete());
         assert_eq!(lesson.progress(), 0.0);
     }
+
+    #[test]
+    fn test_correct_typing_no_typos() {
+        let mut lesson = LessonState::new("abc");
+        input::process_char(&mut lesson, 'a');
+        input::process_char(&mut lesson, 'b');
+        input::process_char(&mut lesson, 'c');
+        assert!(lesson.typo_flags.is_empty());
+        assert_eq!(lesson.accuracy(), 100.0);
+    }
+
+    #[test]
+    fn test_wrong_then_backspace_then_correct_counts_as_error() {
+        let mut lesson = LessonState::new("abc");
+        // Type wrong at pos 0
+        input::process_char(&mut lesson, 'x');
+        assert!(lesson.typo_flags.contains(&0));
+        // Backspace
+        input::process_backspace(&mut lesson);
+        // Typo flag persists
+        assert!(lesson.typo_flags.contains(&0));
+        // Type correct
+        input::process_char(&mut lesson, 'a');
+        assert!(lesson.typo_flags.contains(&0));
+        assert_eq!(lesson.typo_count(), 1);
+        assert!(lesson.accuracy() < 100.0);
+    }
+
+    #[test]
+    fn test_multiple_errors_same_position_counts_as_one() {
+        let mut lesson = LessonState::new("abc");
+        // Wrong, backspace, wrong again, backspace, correct
+        input::process_char(&mut lesson, 'x');
+        input::process_backspace(&mut lesson);
+        input::process_char(&mut lesson, 'y');
+        input::process_backspace(&mut lesson);
+        input::process_char(&mut lesson, 'a');
+        assert_eq!(lesson.typo_count(), 1);
+    }
+
+    #[test]
+    fn test_wrong_char_without_backspace() {
+        let mut lesson = LessonState::new("abc");
+        input::process_char(&mut lesson, 'x'); // wrong at pos 0
+        input::process_char(&mut lesson, 'b'); // correct at pos 1
+        assert_eq!(lesson.typo_count(), 1);
+        assert!(lesson.typo_flags.contains(&0));
+    }
 }