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>

dist/linux/config.toml

@@ -88,6 +88,13 @@ key = "alt+up"
 scope = "global"
 action = { type = "maximize" }
 
+# Skärmbild av arbetsytan (sparas som text + PNG i skärmbilder/)
+[[keybind]]
+key = "alt+p"
+scope = "global"
+label = "Skärmbild"
+action = { type = "screenshot" }
+
 [[keybind]]
 key = "alt+c"
 scope = "global"