refactor: Unify PDF pagination system and rename templates

BREAKING CHANGE: Template names changed from page1_stats.html to page_1.html

## Phase 1: Unified Pagination Function
- Implemented PaginateMultiList() to replace PaginateSkills(), PaginateSpells(), and PaginatePage2PlayLists()
- Single metadata-driven function handles all list types (skills, weapons, spells, equipment)
- Properly handles filters (learned/unlearned/language) via template metadata
- Shares list trackers by ListType+Filter combination to avoid duplication
- Added comprehensive tests for all edge cases

## Phase 2: Template Naming Convention
- Renamed templates to be data-agnostic:
  - page1_stats.html -> page_1.html
  - page1.2_stats.html -> page_1.2.html
  - page2_play.html -> page_2.html
  - page2.2_play.html -> page_2.2.html
  - page3_spell.html -> page_3.html
  - page3.2_spell.html -> page_3.2.html
  - page4_equip.html -> page_4.html
- Updated GenerateContinuationTemplateName() for new naming (page_1.html -> page_1.2.html)
- Updated ExtractBaseTemplateName() to handle new format
- Updated all test files and source files with new template names

## Phase 3: Simplified RenderPageWithContinuations
- Removed hardcoded switch statements based on template names
- Replaced with generic dataMap and unified pagination call
- Extracted populatePageDataFromDistribution() to handle data mapping
- Template type detection now driven by metadata, not hardcoded names

## Benefits
- ✅ Extensibility: Add new templates without code changes
- ✅ Maintainability: One pagination algorithm instead of three
- ✅ Clarity: Template names reflect page numbers, not content types
- ✅ Flexibility: Templates can mix any data types
- ✅ All 40+ tests passing

## Technical Details
- Added SkillsColumn3 and SkillsColumn4 fields to PageData for continuation pages
- Template metadata loaded from HTML comments drives pagination behavior
- Backward compatibility maintained for old template references in comments

docker/Dockerfile.backend.dev

@@ -1,5 +1,4 @@
 # Development Dockerfile für Go Backend mit Live-Reloading
-#FROM golang:1.23-alpine
 FROM golang:1.25-alpine
 
 # Install necessary packages for CGO and SQLite