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>

Cargo.toml

@@ -20,6 +20,7 @@ anyhow = "1"
 image = { version = "0.25", default-features = false, features = ["jpeg", "png", "gif", "bmp", "webp"] }
 interprocess = "2.4"
 unicode-width = "0.2"
+fontdue = "0.9"
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"