Clean up dead code and modernize admin codebase

Created by: jedmund

Summary

This PR modernizes the admin codebase with a comprehensive migration to Svelte 5, improved authentication, and significant code cleanup.

Key Changes

Svelte 5 Migration

• Migrated all components to Svelte 5 runes mode
• Replaced $: reactive statements with $derived() and $effect()
• Updated createEventDispatcher to Svelte 5 callback props
• Modernized event handler syntax

Admin Authentication

• Migrated from basic auth to session-based authentication
• Added authenticated fetch helper for API calls
• Improved security and user experience

Autosave System

• Built new runes-based autosave store
• Integrated autosave across all admin forms (EssayForm, PhotoPostForm, SimplePostForm, ProjectForm)
• Added draft recovery and navigation guards

Server-Side Data Loading

• Migrated lists (projects, posts, media) to server-side loading
• Improved performance and SEO
• Added URL-based filtering and pagination

Code Cleanup

• Removed dead code and unused test pages
• Deleted obsolete components (AdminSegmentedController, BaseSegmentedController, AlbumMetadataPopover)
• Migrated from npm to pnpm

Bug Fixes

• Fixed RSS feed link and formatting rendering
• Fixed drag-and-drop in Edra editor
• Resolved navigation guard infinite loops
• Fixed clickOutside action race conditions

Stats

• 117 files changed
• ~8,000 insertions, ~4,500 deletions
• 62 commits

Test Plan

• Admin login flow works with session auth
• Autosave functions correctly across all forms
• All admin lists load and filter properly
• RSS feeds render correctly
• Drag-and-drop works in editor
• Navigation guards prevent data loss