feat: add quick-save bookmarks and session restore via localStorage

[nancysangani]

Closes #763

Implements client-side project bookmarking and form session persistence — no account or backend changes required.

Changes:

• static/bookmarks.js — DevPathBookmarks module: save/remove/toggle projects to localStorage, restore last-used form values (skills, level, interest, time) on page load, clear on "Clear Filters"
• static/bookmarks.css — bookmark icon alignment on save button, hide empty saved-projects panel, error correlation ID styling
• index.html — two <link> / <script> tags added
• tests/test_bookmarks.py — 35 tests: HTML landmark presence, static file content, API field contract, security headers

[vercel]

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[nancysangani]

Hi @komalharshita, please review the PR when you get a chance. Thanks!