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.lock

@@ -451,12 +451,28 @@ version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
 
+[[package]]
+name = "foldhash"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d9c4f5dac5e15c24eb999c26181a6ca40b39fe946cbe4c263c7209467bc83af2"
+
 [[package]]
 name = "foldhash"
 version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "77ce24cb58228fbb8aa041425bb1050850ac19177686ea6e0f41a70416f56fdb"
 
+[[package]]
+name = "fontdue"
+version = "0.9.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2e57e16b3fe8ff4364c0661fdaac543fb38b29ea9bc9c2f45612d90adf931d2b"
+dependencies = [
+ "hashbrown 0.15.5",
+ "ttf-parser",
+]
+
 [[package]]
 name = "futures-core"
 version = "0.3.32"
@@ -524,6 +540,17 @@ dependencies = [
  "weezl",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.15.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9229cfe53dfd69f0609a49f65461bd93001ea1ef889cd5529dd176593f5338a1"
+dependencies = [
+ "allocator-api2",
+ "equivalent",
+ "foldhash 0.1.5",
+]
+
 [[package]]
 name = "hashbrown"
 version = "0.16.1"
@@ -532,7 +559,7 @@ checksum = "841d1cc9bed7f9236f321df977030373f4a4163ae1a7dbfe1a51a2c1a51d9100"
 dependencies = [
  "allocator-api2",
  "equivalent",
- "foldhash",
+ "foldhash 0.2.0",
 ]
 
 [[package]]
@@ -543,7 +570,7 @@ checksum = "ed5909b6e89a2db4456e54cd5f673791d7eca6732202bbf2a9cc504fe2f9b84a"
 dependencies = [
  "allocator-api2",
  "equivalent",
- "foldhash",
+ "foldhash 0.2.0",
 ]
 
 [[package]]
@@ -1716,12 +1743,19 @@ version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5d99f8c9a7727884afe522e9bd5edbfc91a3312b36a77b5fb8926e4c31a41801"
 
+[[package]]
+name = "ttf-parser"
+version = "0.21.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2c591d83f69777866b9126b24c6dd9a18351f177e49d625920d19f989fd31cf8"
+
 [[package]]
 name = "tui-wm"
 version = "0.1.0"
 dependencies = [
  "anyhow",
  "crossterm",
+ "fontdue",
  "image",
  "interprocess",
  "libc",