Add shift planning, month closure + payroll, and a dark modern redesign

Ports two modules from the friend's Google Apps Script build (kept as
reference in gscript/) onto the TypeScript stack, rewritten cleanly against
this app's own data model rather than copied 1:1:

- Shift planning: weekly template (Sun-Thu evening, Fri/Sat two slots),
  lazy idempotent generation per period (no cron needed), employee
  signup/cancel with collision + capacity checks, admin calendar view with
  slot editing and manual assignment.
- Month closure + payroll: employee confirms the month (blocked while a
  shift is still open), admin locks and finalizes pay (base hours * rate +
  tips/bonus/other), reopen to undo a premature lock, mark paid. Pay rates
  are versioned by date, defaulting the first-ever rate to apply
  retroactively to the employee's whole history.
- A shift left open more than 12h (forgotten clock-out) is auto-closed at
  clock_in + 12h, checked lazily on read instead of a background job.
- Full dark, sharp-edged modern restyle (theme.css replaces tui.css) with
  an amber accent, keeping every existing class name so no component
  logic needed to change.

Backend test coverage (jest) for all three workflows: shift planning,
closure/payroll, and the forgotten-clock-out auto-close.

gscript/.clasp.json

@@ -0,0 +1,16 @@
+{
+  "scriptId": "1O3j68RUOqdnO4N2-sWgv5seES-Bd-AmZDE5HcDjgz0bTN6-jMwNiVrBj",
+  "rootDir": "",
+  "scriptExtensions": [
+    ".js",
+    ".gs"
+  ],
+  "htmlExtensions": [
+    ".html"
+  ],
+  "jsonExtensions": [
+    ".json"
+  ],
+  "filePushOrder": [],
+  "skipSubdirectories": false
+}