feat: add tag color customization with hex picker

- Add nullable color column to tags table with hex validation
- Build ColorPicker component with preset palette and custom hex input
- Apply tag colors to project cards via border styling
- Update all tag API endpoints to handle color field

.sqlx/query-120b55f03836eb7a3f2c3569bc736a07d08b0e44a129e209cdfead695453245e.json

@@ -0,0 +1,46 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "\n        SELECT id, slug, name, color, created_at\n        FROM tags\n        WHERE id = $1\n        ",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "id",
+        "type_info": "Uuid"
+      },
+      {
+        "ordinal": 1,
+        "name": "slug",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 2,
+        "name": "name",
+        "type_info": "Text"
+      },
+      {
+        "ordinal": 3,
+        "name": "color",
+        "type_info": "Varchar"
+      },
+      {
+        "ordinal": 4,
+        "name": "created_at",
+        "type_info": "Timestamptz"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Uuid"
+      ]
+    },
+    "nullable": [
+      false,
+      false,
+      false,
+      true,
+      false
+    ]
+  },
+  "hash": "120b55f03836eb7a3f2c3569bc736a07d08b0e44a129e209cdfead695453245e"
+}