feat: enhance system prompt with app context and dynamic model name (#114)

- Add App Context section describing the left/right panel layout
- Add App Features section with icon locations (history, theme, upload, export, clear)
- Dynamically inject model name into system prompt via {{MODEL_NAME}} placeholder
- Expand edit_diagram tool description with usage guidelines

app/api/chat/route.ts

@@ -282,13 +282,31 @@ Notes:
       },
       edit_diagram: {
         description: `Edit specific parts of the current diagram by replacing exact line matches. Use this tool to make targeted fixes without regenerating the entire XML.
-CRITICAL: Copy-paste the EXACT search pattern from the "Current diagram XML" in system context. Do NOT reorder attributes or reformat - the attribute order in draw.io XML varies and you MUST match it exactly.
-IMPORTANT: Keep edits concise:
-- COPY the exact mxCell line from the current XML (attribute order matters!)
-- Only include the lines that are changing, plus 1-2 surrounding lines for context if needed
-- Break large changes into multiple smaller edits
-- Each search must contain complete lines (never truncate mid-line)
-- First match only - be specific enough to target the right element`,
+
+WHEN TO USE:
+- Changing text labels or values
+- Modifying colors, styles, or visual properties
+- Adding or removing individual elements (1-3 elements)
+- Repositioning specific elements
+- Any small, targeted modification
+
+WHEN TO USE display_diagram INSTEAD:
+- Creating a new diagram from scratch
+- Major restructuring (reorganizing layout, changing diagram type)
+- Adding many new elements (more than 3)
+- After 3 failed edit_diagram attempts
+
+CRITICAL RULES:
+1. Copy-paste the EXACT search pattern from the "Current diagram XML" in system context
+2. Do NOT reorder attributes - attribute order in draw.io XML varies, you MUST match exactly
+3. Always include the element's id attribute for unique targeting
+4. Include complete lines (never truncate mid-line)
+5. For multiple changes, use separate edits in the array
+
+ERROR RECOVERY:
+- If pattern not found, check attribute order matches current XML exactly
+- Retry up to 3 times with adjusted patterns
+- After 3 failures, use display_diagram instead`,
         inputSchema: z.object({
           edits: z.array(z.object({
             search: z.string().describe("EXACT lines copied from current XML (preserve attribute order!)"),