Add Baseline tab, Coverage perf + depth fixes, dark mode toggle

Baseline tab (new):
- 33-feature x 25-partner correctness matrix from Notion Partner Hub
- E3 cell treatment: status emoji + colored letter badges (C/A/I/▶/D/T/✱)
- View/Edit toggle with commit/cancel accumulator (batch saves to PB)
- PocketBase baseline collection with public read/write, SSE live updates
- Stats bar, collapsible categories, fixed legend, 200ms CSS tooltips
- Full-row hover highlight, zebra stripes matching Coverage tab

Coverage tab fixes:
- Depth chip: green = at max achievable (not hardcoded per level)
- maxPossible computed from probe existence (CATALOG_TO_D5_KEY)
- D5 false positives: removed shared key aliases
- Stats bar derives green/amber/red from same logic as depth chips
- Hide badges for non-existent tests (was strikethrough)
- Render perf: memoize featuresByCategory, React.memo CategorySection
- Load perf: getFullList single call (was 7 sequential round-trips)

Both tabs:
- Dark/light/system theme toggle (upper right)
- Column order: Coverage first (1-18), Baseline extras (19-25)
- LangGraph naming (orchestration layer, not model router)
- Row hover highlight including sticky column

Author: jpr5

showcase/pocketbase/pb_migrations/1777700000_create_baseline.js

@@ -0,0 +1,40 @@
+/// <reference path="../pb_data/types.d.ts" />
+migrate(
+  (db) => {
+    const dao = new Dao(db);
+    const c = new Collection({
+      name: "baseline",
+      type: "base",
+      schema: [
+        { name: "key", type: "text", required: true },
+        { name: "partner", type: "text", required: true },
+        { name: "feature", type: "text", required: true },
+        {
+          name: "status",
+          type: "select",
+          required: true,
+          options: { values: ["works", "possible", "impossible", "unknown"], maxSelect: 1 },
+        },
+        { name: "tags", type: "json", options: { maxSize: 500 } },
+        { name: "updated_at", type: "date", required: true },
+        { name: "updated_by", type: "text" },
+      ],
+      indexes: [
+        "CREATE UNIQUE INDEX idx_baseline_key ON baseline (key)",
+        "CREATE INDEX idx_baseline_partner ON baseline (partner)",
+        "CREATE INDEX idx_baseline_feature ON baseline (feature)",
+      ],
+      listRule: "",
+      viewRule: "",
+      createRule: "",
+      updateRule: "",
+      deleteRule: null,
+    });
+    dao.saveCollection(c);
+  },
+  (db) => {
+    const dao = new Dao(db);
+    const c = dao.findCollectionByNameOrId("baseline");
+    dao.deleteCollection(c);
+  },
+);