Screenshot feature (text + PNG) and keybind editor tab in settings

- New screenshot action (default alt+p, bindable like any action):
  renders the current workspace to a buffer and saves it as plain text
  and/or PNG (screenshot_format = text|png|both, screenshot_dir
  configurable, default skärmbilder/ under the config dir). The PNG is
  rasterized cell by cell with an embedded DejaVu Sans Mono font
  (vendored, ~340 kB); glyphs missing from the font (color emoji)
  render as filled boxes. A popup confirms the saved paths
- Settings dialog gains a 'Tangenter' row: lists all keybinds with
  label and scope, Enter captures the next keypress as the new chord
  (persisted by rewriting [[keybind]] via toml_edit), Delete removes
  a binding — changes take effect immediately
- Integration suite: 48/48 (screenshot files + validity, keybind list,
  live rebind ctrl+space→alt+z)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/log.rs

@@ -65,7 +65,7 @@ fn today_filename() -> String {
     format!("{:04}-{:02}-{:02}.log", y, mo, d)
 }
 
-fn timestamp() -> String {
+pub fn timestamp() -> String {
     let secs = SystemTime::now()
         .duration_since(SystemTime::UNIX_EPOCH)
         .map(|d| d.as_secs())