Reduce to a pure on/off extension

Durations were the only thing keeping a lot of machinery alive, so drop
them and let the dead code go with them.

Removed:
- The "Caffeinate for a Duration" command.
- lib/duration.ts entirely (parseDuration / formatDuration /
  formatRemaining / normalizeDuration / DURATION_PRESETS) plus its test
  file. Every export was reachable only from duration paths.
- The 30s setInterval re-render in the status view. It existed only to
  keep a "27m left" countdown honest, and with no timed sessions there is
  nothing to count down.
- The duration-preset actions in the status view.
- describeOptions(), which only the duration form used to explain itself.
- durationSeconds / expiresAt from the options, state and command
  builder, along with the `-t <seconds>` and `sleep <n>` branches. Both
  platforms now hold the assertion until signalled, which is what "on"
  means here.
- The unused "restarted" member of ToggleResult.

The status view now reports "since <time>" instead of a countdown, and
keeps a Refresh action.

Also fixes a type error the narrower helper surfaced: the status view's
run() helper was typed () => Promise<void> while startCaffeination
returns Promise<CaffeinateState> and stopCaffeination returns
Promise<boolean>.

407 deletions against 57 insertions.

Verified: vici lint, tsc --noEmit, eslint, 8/8 vitest, vici build, and
live in Vicinae — toggle on spawns `caffeinate -i`, toggle off stops it,
and a pre-existing user-owned `caffeinate -u -dmi` is left untouched
throughout.

package.json

@@ -21,12 +21,6 @@
       "title": "Caffeination Status",
       "description": "See whether sleep is currently being prevented, and for how much longer. Start or stop it from here.",
       "mode": "view"
-    },
-    {
-      "name": "caffeinate-for",
-      "title": "Caffeinate for a Duration",
-      "description": "Keep your computer awake for a set amount of time, then let it sleep again automatically.",
-      "mode": "view"
     }
   ],
   "preferences": [
@@ -74,4 +68,4 @@
     "test": "vitest run",
     "test:watch": "vitest"
   }
-}
+}