Seed default config on install; share defaults JSON with runtime

The defaults now live in a single file, scripts/config.default.json:

- src/config.ts imports it via 'with { type: "json" }' and derives
  DEFAULT_CONFIG (with seconds->ms conversion at the boundary), so the
  CLI help text always matches what the seeded user config contains.
- scripts/install.sh copies the same file to
  $XDG_CONFIG_HOME/move/config.json only if no file is already there.
  Existing configs (yours or from a previous install) are never
  overwritten.
- scripts/uninstall.sh does not touch the user config file at all,
  following the strict Unix convention. It does print a one-line
  notice pointing at the path so the user can rm -rf the dir
  themselves if they want a fully-clean removal.
- tsconfig.json gains resolveJsonModule:true so tsc accepts the JSON
  import.
- A small runtime assertion in src/config.ts (assertSeedShape) fails
  loudly if the seed file is missing keys or has wrong types.
- README Configuration section documents the seed behavior; Uninstall
  section documents the leave-config-behind policy with the rm-it-
  yourself command; Files table gains scripts/config.default.json.

src/config.ts

@@ -21,6 +21,13 @@
 
 import { join } from "node:path";
 
+// Single source of truth for default values. The same file ships in the
+// install tree and is copied to $XDG_CONFIG_HOME/move/config.json on a
+// fresh install (only if no config exists there yet). Values use the CLI
+// units (seconds for time fields, ms for stepDelay, pixels for stepCount);
+// the seconds->ms conversion happens below where DEFAULT_CONFIG is built.
+import seedRaw from "../scripts/config.default.json" with { type: "json" };
+
 /**
  * The shape of a resolved runtime configuration. `readonly` to make
  * accidental mutation a type error.
@@ -44,16 +51,52 @@ export interface Config {
     readonly verbose: boolean;
 }
 
+/**
+ * Shape of `scripts/config.default.json` after parsing. The cast below
+ * trusts the file's structure; `assertSeedShape` performs a small runtime
+ * sanity check at import time so a corrupted seed file fails loudly
+ * instead of silently producing `NaN` or `undefined` defaults.
+ */
+interface SeedShape {
+    moveInterval: number;   // seconds
+    checkInterval: number;  // seconds
+    stepDelay: number;      // milliseconds
+    stepCount: number;      // pixels
+    verbose: boolean;
+}
+
+function assertSeedShape(raw: unknown): asserts raw is SeedShape {
+    if (typeof raw !== "object" || raw === null) {
+        throw new Error("scripts/config.default.json: root must be an object");
+    }
+    const r = raw as Record<string, unknown>;
+    for (const key of ["moveInterval", "checkInterval", "stepDelay", "stepCount"] as const) {
+        const v = r[key];
+        if (typeof v !== "number" || !Number.isFinite(v) || v <= 0) {
+            throw new Error(`scripts/config.default.json: '${key}' must be a positive finite number (got ${JSON.stringify(v)})`);
+        }
+    }
+    if (typeof r.verbose !== "boolean") {
+        throw new Error(`scripts/config.default.json: 'verbose' must be a boolean (got ${JSON.stringify(r.verbose)})`);
+    }
+}
+
+assertSeedShape(seedRaw);
+const seed: SeedShape = seedRaw;
+
 /**
  * Built-in defaults used when neither the CLI nor the config file supplies
- * a value for a given field.
+ * a value for a given field. Derived from `scripts/config.default.json`
+ * (the single source of truth); time-valued fields are converted from
+ * seconds to milliseconds here so the rest of the codebase works in
+ * internal units.
  */
 export const DEFAULT_CONFIG: Config = {
-    moveInterval: 4 * 60 * 1000, // 4 minutes in milliseconds
-    checkInterval: 10 * 1000,    // Check every 10 seconds
-    stepDelay: 50,               // ms between mouse steps
-    stepCount: 250,              // pixels to move per sweep
-    verbose: false,              // quiet by default; startup banner still prints
+    moveInterval:  seed.moveInterval * 1000,
+    checkInterval: seed.checkInterval * 1000,
+    stepDelay:     seed.stepDelay,
+    stepCount:     seed.stepCount,
+    verbose:       seed.verbose,
 };
 
 /**