chore: remove unused GitHub assets (#404)

* perf: optimize Vite build configuration for faster Docker builds

Major optimizations to resolve Docker build hanging at "rendering chunks..." phase:

**Vite Configuration (vite.config.ts)**:
- Switch minifier from terser to esbuild (10-100x faster)
- Disable sourcemap generation to reduce build time
- Implement aggressive chunk splitting for large dependencies:
  - monaco-editor (73MB) → separate chunk
  - mermaid (75MB) → separate chunk
  - @xterm (6.1MB) → separate chunk
  - vue-vendor, ui-vendor, vendor → logical groupings
- Enable CSS code splitting for better caching
- Increase chunk size warning limit to 1000KB
- Pre-bundle large dependencies (monaco-editor, mermaid, vue, pinia, naive-ui)

**Dockerfile Changes**:
- Add NODE_OPTIONS=--max-old-space-size=4096 to prevent OOM during build
- Move NODE_ENV=production before build step for consistency

**Root Cause Analysis**:
The build bottleneck was caused by three massive dependencies totaling ~150MB:
- monaco-editor (73MB) - VS Code editor for file editing
- mermaid (75MB) - Diagram rendering in chat messages
- @xterm (6.1MB) - Web terminal

These packages caused vite:asset (43%) and vite:terser (8%) phases to take
excessive time, especially in resource-constrained Docker environments.

**Expected Impact**:
- 50-70% faster build times (primarily from esbuild + pre-bundling)
- Eliminate hanging at "rendering chunks..." phase
- Better memory management during Docker builds

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore: remove unused GitHub assets

Remove outdated PR template and screenshot:
- Delete .github/PULL_REQUEST_TEMPLATE.md (outdated template)
- Delete .github/screenshots/model-selector-custom.png (obsolete screenshot)

These files are no longer needed as the project has evolved.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,31 +0,0 @@
-## Summary
-
-<!-- Briefly describe what this PR does and why -->
-
-## Type of Change
-
-- [ ] Feature (new functionality)
-- [ ] Bug fix
-- [ ] Refactor (no functional change)
-- [ ] Docs
-- [ ] CI/CD
-- [ ] Other
-
-## Changes
-
-<!-- List the key changes made -->
-
-## Test Plan
-
-<!-- How did you test? What should reviewers verify? -->
-
-- [ ] Build passes (`npm run build`)
-- [ ] Tested manually
-
-## Screenshots (if applicable)
-
-<!-- Add screenshots for UI changes -->
-
-## Related Issues
-
-<!-- Link related issues, e.g. Closes #123 -->