refactor(semantics): migrate legacy @TIER to @COMPLEXITY annotations

- Replaced @TIER: TRIVIAL with @COMPLEXITY: 1
- Replaced @TIER: STANDARD with @COMPLEXITY: 3
- Replaced @TIER: CRITICAL with @COMPLEXITY: 5
- Manually elevated specific critical/complex components to levels 2 and 4
- Ignored legacy, specs, and node_modules directories
- Updated generated semantic map

frontend/src/lib/ui/PageHeader.svelte

@@ -1,6 +1,6 @@
 <!-- [DEF:PageHeader:Component] -->
 <!--
-@TIER: TRIVIAL
+@COMPLEXITY: 1
 @SEMANTICS: page-header, layout-atom
 @PURPOSE:   Standardized page header with title and action area.
 @LAYER:     Atom