fix: make the template workflow real, and the UI legible

The headline flow — fill a config from its template — did not work. Three
bugs, each individually enough to make the tool feel unfinished:

- `add_missing` was unreachable. `find_binding` returned the first match in
  a linear array and `edit_append` was also bound to `a`, 17 entries earlier.
  Bindings now resolve through one table built from the config, and
  `KeybindsConfig::conflicts()` reports any key bound twice instead of
  letting one silently shadow the other. `add_missing` moves to `<Space>`,
  with `A` to take every missing default at once.
- Item status was ignored on write. Every writer fell back to
  `template_value`, so "(missing)" keys landed in the output regardless and
  saving was close to a copy. Unaccepted entries are now omitted.
- Parse errors were swallowed, then the good file was overwritten. A
  malformed target opened as an empty editor and `:w` truncated it. Parse
  failures are now fatal, reported with context, before the TUI starts.

Also:

- Split "differs from the template" (ItemStatus::Overridden) from "you
  edited this" (ConfigItem::dirty). A real config differs from its example
  on nearly every line, so the old Modified flag painted the whole list
  orange and carried no signal.
- Splice template-only entries in at their natural position instead of
  appending them, so a missing nested key no longer strands itself at the
  bottom of the tree at depth 3.
- Preserve .env comments and blank lines across a save.
- Header naming the output file and the template it is compared against,
  plus missing/changed counts. Previously nothing on screen said which
  files were in play or where `:w` would write.
- `?` opens a two-column keybinding reference built from the user's own
  config, scrollable on short terminals. The status bar drops to a few
  contextual hints; the old one joined 11+ hints into an unwrapped line
  that truncated at 80 columns.
- Foldable groups, which the `+ ` prefix had always implied.
- Search covers values as well as keys, reports `match n/m`, and `Esc`
  restores the pre-search cursor.
- Quitting with unsaved changes is refused; added `:q!`, `:x`, `:w <path>`.
- Status messages expire after 4s instead of permanently replacing hints.
- Cancelling a rename on a just-created item removes it again.
- Editing a Number field with text demotes the type instead of failing
  obscurely at write time.
- Undo-tree branches are reachable via `g-`/`g+` and reported on undo.
- Poll instead of blocking, so resizes repaint; drop the unused mouse
  capture that was costing terminal text selection for nothing.
- `--format` accepts xml, ini and properties, which were already supported.

36 tests, up from 13.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01C5mrbEFmwv2A4cdzbcSiS1

src/format/properties.rs

@@ -1,4 +1,4 @@
-use super::{ConfigItem, FormatHandler, ItemStatus, ValueType, PathSegment};
+use super::{ConfigItem, FormatHandler, ValueType, PathSegment};
 use java_properties::{LineContent, PropertiesIter, PropertiesWriter};
 use std::fs::File;
 use std::io::{BufReader, BufWriter};
@@ -27,17 +27,7 @@ impl FormatHandler for PropertiesHandler {
                     current_path.push(PathSegment::Key(part.to_string()));
                     
                     if groups.insert(current_path.clone()) {
-                        vars.push(ConfigItem {
-                            key: part.to_string(),
-                            path: current_path.clone(),
-                            value: None,
-                            template_value: None,
-                            default_value: None,
-                            depth: i,
-                            is_group: true,
-                            status: ItemStatus::Present,
-                            value_type: ValueType::Null,
-                        });
+                        vars.push(ConfigItem::group(*part, current_path.clone(), i));
                     }
                 }
 
@@ -45,17 +35,13 @@ impl FormatHandler for PropertiesHandler {
                 let last_key = parts.last().unwrap_or(&"").to_string();
                 final_path.push(PathSegment::Key(last_key.clone()));
 
-                vars.push(ConfigItem {
-                    key: last_key,
-                    path: final_path,
-                    value: Some(value.clone()),
-                    template_value: Some(value.clone()),
-                    default_value: Some(value.clone()),
-                    depth: parts.len().saturating_sub(1),
-                    is_group: false,
-                    status: ItemStatus::Present,
-                    value_type: ValueType::String,
-                });
+                vars.push(ConfigItem::leaf(
+                    last_key,
+                    final_path,
+                    value,
+                    parts.len().saturating_sub(1),
+                    ValueType::String,
+                ));
             }
         }
 
@@ -69,11 +55,8 @@ impl FormatHandler for PropertiesHandler {
         let mut prop_writer = PropertiesWriter::new(writer);
         
         for var in vars {
-            if !var.is_group {
-                let val = var.value.as_deref()
-                    .or(var.template_value.as_deref())
-                    .unwrap_or("");
-                prop_writer.write(&var.path_string(), val)?;
+            if var.should_write() {
+                prop_writer.write(&var.path_string(), var.write_value())?;
             }
         }
 
@@ -86,34 +69,19 @@ impl FormatHandler for PropertiesHandler {
 mod tests {
     use super::*;
     use tempfile::NamedTempFile;
-    use std::io::Write;
 
     #[test]
     fn test_group_rename_write() {
         let handler = PropertiesHandler;
         let mut vars = vec![
-            ConfigItem {
-                key: "server".to_string(),
-                path: vec![PathSegment::Key("server".to_string())],
-                value: None,
-                template_value: None,
-                default_value: None,
-                depth: 0,
-                is_group: true,
-                status: ItemStatus::Present,
-                value_type: ValueType::Null,
-            },
-            ConfigItem {
-                key: "port".to_string(),
-                path: vec![PathSegment::Key("server".to_string()), PathSegment::Key("port".to_string())],
-                value: Some("8080".to_string()),
-                template_value: Some("8080".to_string()),
-                default_value: Some("8080".to_string()),
-                depth: 1,
-                is_group: false,
-                status: ItemStatus::Present,
-                value_type: ValueType::String,
-            }
+            ConfigItem::group("server", vec![PathSegment::Key("server".to_string())], 0),
+            ConfigItem::leaf(
+                "port",
+                vec![PathSegment::Key("server".to_string()), PathSegment::Key("port".to_string())],
+                "8080",
+                1,
+                ValueType::String,
+            ),
         ];
 
         // Rename "server" to "srv"