Add today-view plugin with Tana, ICS, and Donetick data sources

Implements an e-ink daily dashboard plugin ("today") with four sections:
date/focus/success header, agenda timeline, chores checklist, and
due/overdue task lists.

Data sources:
- Focus & success text: Tana daily note (src/sources/tana.ts)
- Due/overdue tasks: Tana task search (src/sources/tana.ts)
- Agenda events: ICS calendar feeds (src/sources/ics.ts)
- Chores: Donetick API (src/sources/donetick.ts)

All sources fetch in parallel and fall back gracefully on error.
Tests use mock HTTP servers with synthetic data — no real services needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config.sample.json

@@ -0,0 +1,26 @@
+{
+  "base_url": "http://192.168.1.100:2300",
+  "devices": {
+    "AA:BB:CC:DD:EE:FF": {
+      "plugin": "today",
+      "settings": {
+        "calendar_urls": [
+          "https://calendar.example.com/feed.ics"
+        ],
+        "tana": {
+          "url": "http://192.168.1.100:8262",
+          "token": "your-tana-api-token",
+          "workspace": "your-workspace-id",
+          "task_tag_id": "your-task-tag-node-id",
+          "due_date_field_id": "your-due-date-field-id"
+        },
+        "donetick": {
+          "token": "your-donetick-access-token",
+          "user_id": 1
+        }
+      },
+      "refresh": 300,
+      "model": "inkplate_10"
+    }
+  }
+}