feat: admin page with dynamic converter loading

Implements full admin interface for managing units and groups.
Migrates converter from static imports to server-side data loading.

- Switch adapter-static → adapter-node
- Add bcryptjs session auth with HMAC-signed cookies
- Add data/units.json and data/config.json data layer
- Add atomic file writes via temp-file rename
- Add public GET /api/units endpoint
- Add auth-gated admin CRUD API for units and groups
- Add two-panel admin UI with group tree and edit forms
- Add YOLO mode toggle for cross-group conversions
- Add visual group dividers in converter results grid
- Update ResultItem type for nullable convertedValue and isYolo flag
- Group deletion supports reassign/orphan with toBase recalculation

Rollback point: 067fd44

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/admin/api/groups/[id]/+server.ts

@@ -0,0 +1,111 @@
+import { json } from '@sveltejs/kit';
+import { loadData, saveData, loadConfig } from '$lib/server/data';
+import { requireAuth } from '$lib/server/auth';
+import type { RequestHandler } from './$types';
+import type { Group } from '$lib/types';
+
+function auth(request: Request) {
+  return requireAuth(request, loadConfig());
+}
+
+export const PUT: RequestHandler = async ({ request, params }) => {
+  if (!auth(request)) return json({ error: 'Unauthorized' }, { status: 401 });
+
+  const { id } = params;
+  const body = await request.json().catch(() => null);
+  if (!body) return json({ error: 'Invalid JSON' }, { status: 400 });
+
+  const data = loadData();
+  const gIdx = data.groups.findIndex((g) => g.id === id);
+  if (gIdx === -1) return json({ error: 'Group not found' }, { status: 404 });
+
+  const patch = body as Partial<Group>;
+  const currentGroup = data.groups[gIdx];
+
+  // Special case: changing baseUnitId requires recalculating all unit toBase values
+  if (patch.baseUnitId !== undefined && patch.baseUnitId !== currentGroup.baseUnitId) {
+    const newBaseId = patch.baseUnitId;
+    const newBaseUnit = data.units.find((u) => u.id === newBaseId && u.group === id);
+    if (!newBaseUnit) {
+      return json({ error: `baseUnitId '${newBaseId}' not found in this group` }, { status: 400 });
+    }
+
+    const X = newBaseUnit.toBase; // Current toBase of the new base unit
+    // Recalculate all units in this group: unit.toBase = unit.toBase / X
+    // Then set new base unit toBase = 1.0
+    for (const unit of data.units) {
+      if (unit.group === id) {
+        if (unit.id === newBaseId) {
+          unit.toBase = 1.0;
+        } else {
+          unit.toBase = unit.toBase / X;
+        }
+      }
+    }
+  }
+
+  const updated: Group = { ...currentGroup, ...patch };
+  data.groups[gIdx] = updated;
+  saveData(data);
+
+  return json(updated);
+};
+
+export const DELETE: RequestHandler = async ({ request, params }) => {
+  if (!auth(request)) return json({ error: 'Unauthorized' }, { status: 401 });
+
+  const { id } = params;
+  const body = await request.json().catch(() => ({}));
+  const { action, targetGroupId, toBaseAction } = body as {
+    action?: 'reassign' | 'orphan';
+    targetGroupId?: string;
+    toBaseAction?: 'recalculate' | 'reset' | 'keep';
+  };
+
+  if (!action || (action !== 'reassign' && action !== 'orphan')) {
+    return json({ error: "action must be 'reassign' or 'orphan'" }, { status: 400 });
+  }
+
+  const data = loadData();
+  const gIdx = data.groups.findIndex((g) => g.id === id);
+  if (gIdx === -1) return json({ error: 'Group not found' }, { status: 404 });
+
+  const fromGroup = data.groups[gIdx];
+  const unitsInGroup = data.units.filter((u) => u.group === id);
+
+  if (action === 'reassign') {
+    if (!targetGroupId) {
+      return json({ error: 'targetGroupId required for reassign' }, { status: 400 });
+    }
+    if (targetGroupId === id) {
+      return json({ error: 'Cannot reassign to the same group' }, { status: 400 });
+    }
+    const toGroup = data.groups.find((g) => g.id === targetGroupId);
+    if (!toGroup) {
+      return json({ error: `Target group '${targetGroupId}' not found` }, { status: 404 });
+    }
+
+    for (const unit of data.units) {
+      if (unit.group === id) {
+        if (toBaseAction === 'recalculate') {
+          unit.toBase = (unit.toBase * fromGroup.toUniversal) / toGroup.toUniversal;
+        } else if (toBaseAction === 'reset') {
+          unit.toBase = 1.0;
+        }
+        // 'keep' or default: no change to toBase
+        unit.group = targetGroupId;
+      }
+    }
+  } else if (action === 'orphan') {
+    for (const unit of data.units) {
+      if (unit.group === id) {
+        unit.group = null;
+      }
+    }
+  }
+
+  data.groups.splice(gIdx, 1);
+  saveData(data);
+
+  return json({ ok: true, unitsAffected: unitsInGroup.length });
+};