feat: integrate HTML template engine with theme selection

- Add theme configuration to WebServer struct
- Create default theme with base.html layout template
- Update content.go to apply theme templates to generated HTML
- Output is now full HTML documents with styling

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

PLAN.md

@@ -10,7 +10,7 @@
 ## Current Status
 - [x] Project backbone exists (HTTP server, graceful shutdown, config reading).
 - [x] Markdown parsing logic implemented (using goldmark).
-- [ ] HTML Template engine integrated (Hugo-like theme selection).
+- [x] HTML Template engine integrated (Hugo-like theme selection).
 - [ ] PDF Generation implemented (Pure Go library selected and integrated).
 - [x] CLI Mode (`gocv`) generates static files to `./output` and exits.
 - [ ] Serve Mode (`gocv serve`) hosts HTML and serves PDF on demand.
@@ -19,11 +19,12 @@
 
 ## Active Task
 - [x] Analyze existing backbone code and integrate Markdown parsing.
-- [ ] Integrate HTML template engine with theme selection.
+- [x] Integrate HTML template engine with theme selection.
+- [ ] Implement Serve Mode with file watching for live reload.
 
 ## Known Issues / Blockers
 - [ ] Identify best Pure Go PDF library that supports HTML/CSS (or define CSS subset).
-- [ ] Current output is raw HTML fragments without full HTML document structure.
+- [x] Current output is raw HTML fragments without full HTML document structure.
 
 ## Completed Log
 - [x] Initial project structure defined.
@@ -31,3 +32,6 @@
 - [x] CLI mode vs Serve mode distinction implemented (checks os.Args[1] for "serve").
 - [x] Content reading from `./content` directory implemented.
 - [x] Markdown to HTML conversion using goldmark library.
+- [x] Theme system with templates/themes/{theme}/base.html structure.
+- [x] Config.yaml theme setting (defaults to "default").
+- [x] Output now generates complete HTML documents with styling.