feat: add localStorage bookmarking system with session-based recently viewed projects

[divyanshim27]

Summary

Implements the two Advanced-level open issues listed in the README: a client-side
bookmarking system and server-side "recently viewed" tracking. No database changes
required — bookmarks live in localStorage, recently viewed uses Flask's built-in
session.

Closes #1230

What Changed

Bookmarking (client-side, static/script.js)

• Added BookmarkManager object handling get/add/remove/isBookmarked against localStorage
• Capped at 10 bookmarks (oldest evicted first — simple LRU)
• Duplicate-save prevention
• Wrapped JSON.parse in try/catch to recover gracefully from corrupted/hand-edited localStorage instead of crashing the page
• Added a bookmark toggle button (#bookmark-btn) to templates/project.html, with aria-pressed state kept in sync for screen readers
• New /bookmarks route + templates/bookmarks.html rendering all saved projects client-side

Recently Viewed (server-side, routes/main_routes.py)

• /project/<int:project_id> now records the visit in session["recently_viewed"], capped at the last 5, most-recent-first
• Fixed a de-dup edge case from the original proposal: re-visiting an already-viewed project now correctly moves it back to the front of the list instead of being ignored
• New GET /api/recently-viewed endpoint returning full project objects for the session's recently viewed IDs
• New "Recently Viewed" section on the homepage, populated via fetch("/api/recently-viewed") and hidden when empty

Other

• Added app.secret_key config (required for Flask sessions) — sourced from SECRET_KEY env var with a dev-only fallback; documented in .env.example
• README updated with a short description of both features

Files Changed

• static/script.js — BookmarkManager, bookmark button init, homepage recently-viewed fetch
• templates/project.html — bookmark button markup
• templates/bookmarks.html — new
• templates/index.html — recently-viewed section
• routes/main_routes.py — session tracking + /api/recently-viewed
• app.py — secret_key config
• .env.example — SECRET_KEY documented
• static/style.css — minor bookmark button styling
• tests/test_basic.py — 4 new tests
• README.md — feature docs

Acceptance Criteria

• BookmarkManager implemented in static/script.js using localStorage
• Bookmark toggle button on every project detail page
• Bookmarked state persists across browser sessions (localStorage, not session-scoped)
• /bookmarks route and template showing all saved projects
• Recently viewed tracked server-side via Flask session (last 5 projects)
• Recently viewed section appears on homepage after first project visit
• Built-in Flask session used — no database required
• Backend tests added for recently-viewed logic (see note below on bookmark JS tests)

Testing

Automated (python tests/test_basic.py):

• test_recently_viewed_tracks_last_five
• test_recently_viewed_revisit_moves_to_front
• test_bookmarks_page_loads
• test_project_detail_404_for_invalid_id

All existing tests still pass.

Manual QA — bookmark add/remove/dedup logic is pure client-side localStorage
state with no server round-trip, so it isn't covered by the Flask test client. This
repo doesn't currently have a JS test runner configured (no Jest/Vitest in
package.json or equivalent), so I verified manually:

• Save a project → refresh page → button still shows "Saved"
• Save an 11th project → oldest bookmark evicted
• Save the same project twice → only 1 entry created
• Visit /bookmarks with 0 saved → empty state message shown
• Visit /bookmarks with saved projects → cards render correctly
• Visit 6 different projects → homepage shows last 5, most recently viewed first
• Re-visit an already-viewed project → moves to front of recently-viewed list

Open to adding a Jest setup in a follow-up if

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