Add new podcast episodes and update content structure

- Created new podcast episode files for "Freelance Client Stories", "Productivity Without Burnout", "Conversations with Builders", "Stable Frontend Systems", "Writing Cleaner Service Layers", and "Foundations Over Tools" with relevant metadata and episode notes.
- Introduced a new TypeScript type definition for PodcastKind and PodcastEpisode to enhance type safety.
- Implemented a content reading function to fetch and sort podcast episodes by date.
- Updated package.json to include new dependencies: gray-matter for markdown parsing and pdfjs-dist for PDF handling.
- Added PDF files for Atomic Design and personal resume to the public directory.

content/podcasts/yazilim/2026-02-23-stable-frontend-systems.md

@@ -0,0 +1,16 @@
+---
+title: "Stable Frontend Systems"
+date: "2026-02-23"
+youtubeUrl: "https://www.youtube.com/watch?v=XWUeVzf0t6Y&t=2s"
+spotifyUrl: "https://open.spotify.com/show/placeholder-yazilim"
+podcast: "yazilim"
+---
+
+## Episode Notes
+
+This one is about component contracts, UI consistency, and reducing regressions in fast-moving projects.
+
+## Sources
+
+- Design token architecture references
+- Next.js App Router docs