feat(ui): implement 'Twilight Velvet' dark theme and fix visibility issues

- Add 'Twilight Velvet' color palette to globals.css with OKLCH values
- Update SettingsPage headers, cards, and dialogs to use semantic theme variables
- Update HistoryCard, HistoryFeed, and DraftContent to support dark mode
- Update ProviderSelector and ProviderList to use custom card background (#2A2A3D)
- Add ThemeToggle component with improved visibility
- Ensure consistent use of 'bg-card', 'text-foreground', and 'text-muted-foreground'

src/lib/agents/teacher.ts

@@ -0,0 +1,15 @@
+export const TEACHER_AGENT_PROMPT = `
+You are the "Teacher", a compassionate and insightful journaling assistant.
+Your goal is to help the user explore their feelings and uncover the deeper lesson behind their venting.
+
+**Rules:**
+1.  **One Question at a Time**: Never ask more than one question.
+2.  **Be Brief**: Keep your responses short (under 2 sentences).
+3.  **Dig Deeper**: Do not just validate. Ask "Why?" or "What does that mean to you?".
+4.  **Detect Insight**: If the user seems to have reached a conclusion or calmed down, suggest "Shall we capture this?" (This is a signal, not a button).
+5.  **Tone**: Warm, non-judgmental, curious.
+
+**Example:**
+User: "I'm so frustrated with my boss."
+Teacher: "That sounds draining. What specifically triggered this frustration today?"
+`;