fix: Mobile hamburger menu and iPad sidebar visibility

Mobile fixes:
- Add click toggle handler for hamburger menu (was hover-only)
- Menu now opens/closes on tap and closes when clicking outside
- Keep hover support for desktop

iPad fixes:
- Sidebar content now visible on touch devices (901-1280px)
- Added (hover: hover) media query to prevent hide-on-hover on tablets

Security improvements:
- Replace exec.CommandContext with go-git library for git operations
- Add path traversal and command injection prevention
- Fix race condition in template hot reload
- Add environment-based cookie Secure flag

Code quality:
- Add constants.go for magic numbers
- Remove unused code (ParsePreferenceToggleRequest, DomainError)
- Add FOUC prevention with inline critical CSS
- Add Makefile dev/run/clean targets
- Fix README git clone URL
- Add doc/DECISIONS.md for architectural decisions

Tests:
- Add hamburger menu click toggle tests
- Add iPad sidebar visibility tests
- Update security tests for go-git implementation
- Add cookie Secure flag tests

internal/handlers/types.go

@@ -84,21 +84,6 @@ func ParsePDFExportRequest(r *http.Request) (*PDFExportRequest, error) {
 	return req, nil
 }
 
-// PreferenceToggleRequest represents a toggle request with language context
-type PreferenceToggleRequest struct {
-	Lang string // Current language from query or cookie
-}
-
-// ParsePreferenceToggleRequest parses toggle request parameters
-func ParsePreferenceToggleRequest(r *http.Request, defaultLang string) *PreferenceToggleRequest {
-	lang := r.URL.Query().Get("lang")
-	if lang == "" {
-		lang = defaultLang
-	}
-
-	return &PreferenceToggleRequest{Lang: lang}
-}
-
 // ==============================================================================
 // RESPONSE TYPES
 // Structured response types for consistent API responses