Add the UI foundation: vim keymap, widgets and the component contract

- keymap: every key sequence in vim notation (gg, yy, <C-a>,
  <Space>s), with counts (5j), per-screen overrides and which-key hints,
  all from the same tables.
- Component: the one trait for screens and popups. They only look at
  the data through a Context and answer with Actions.
- Action: everything a screen can ask the app to do.
- widgets: big digits, buttons, a day timeline, a heatmap, a text field.
- ui/mod.rs: tabs, status line and the which-key popup; terminal.rs sets
  the terminal up (and restores it after a crash).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/action.rs

@@ -0,0 +1,85 @@
+//! Everything a screen or popup can ask the app to do.
+//!
+//! Screens and popups never change your data themselves: they return an
+//! `Action`, and the app applies it in one place (`App::apply`), saves, and
+//! reports what happened. That's the only way anything changes.
+
+use jiff::civil::Date;
+use jiff::{SignedDuration, Timestamp};
+
+use crate::domain::Segment;
+use crate::export::ExportFormat;
+use crate::export::timesheet::CopyPreset;
+use crate::time::DateRange;
+use crate::ui::{Component, Tab};
+
+pub enum Action {
+    // ----- Moving around the app
+    GoTo(Tab),
+    OpenPopup(Box<dyn Component>),
+    ClosePopup,
+    /// The help popup for the current screen.
+    ShowHelp,
+    /// Show a message in the status line.
+    Notify(String),
+    Quit,
+
+    // ----- The shift
+    Start,
+    /// Start working as if you had pressed Start at this earlier moment.
+    StartAt(Timestamp),
+    Pause,
+    /// Pause as if you had pressed Pause at this earlier moment.
+    PauseAt(Timestamp),
+    EndDay,
+    EndDayAt(Timestamp),
+    /// Move the start of the session in progress by this much.
+    ShiftRunningStart(SignedDuration),
+    /// Correct the session in progress: when it started, and optionally
+    /// when it actually stopped (you forgot to pause).
+    FixRunning {
+        since: Timestamp,
+        stopped: Option<Timestamp>,
+    },
+    /// Silence the break suggestion for a while.
+    Snooze,
+
+    // ----- Recorded work
+    AddSegment {
+        day: Date,
+        segment: Segment,
+    },
+    /// Replace segment `index` of `day`, filing the new one under `new_day`.
+    ReplaceSegment {
+        day: Date,
+        index: usize,
+        segment: Segment,
+        new_day: Date,
+    },
+    DeleteSegment {
+        day: Date,
+        index: usize,
+    },
+    Undo,
+    Redo,
+
+    // ----- Getting data out
+    Copy {
+        days: Vec<Date>,
+        preset: CopyPreset,
+    },
+    Export {
+        format: ExportFormat,
+        range: DateRange,
+    },
+
+    // ----- Settings, by their key in config.toml
+    SetSetting {
+        key: &'static str,
+        value: String,
+    },
+    ResetSetting {
+        key: &'static str,
+    },
+    ToggleWhichKey,
+}