Complete Direction B design migration, fix profile/notification bugs

[aniketpatidar]

Summary

app/views/
├── categories/, posts/, admin/, notifications/, users/, discussions/   # Direction B dark theme applied
├── layouts/_sidebar.html.erb        # Notifications moved from navbar bell to regular sidebar link
└── nav/_notifications.html.erb      # rewritten: icon+label+unread badge, matches other nav items

app/helpers/application_helper.rb    # upvote_icon/downvote_icon guard against nil user
app/views/discussions/posts/_post.html.erb   # current_user rescued (Turbo Stream broadcasts have no Warden session)
app/views/users/show.html.erb        # fix: truncate(post.body) -> truncate(post.body.to_plain_text)

app/models/mock*.rb, app/controllers/mocks*, app/views/mocks/   # Timed Mock feature wired end-to-end

Evidence

• Before: bin/rails test → 4 failures, 1 error (stale leaderboard assertions, old chip markup, Devise/Warden crash on broadcast render)

• After: bin/rails test → 134 runs, 302 assertions, 0 failures, 0 errors

• Before: Notifications bell icon only, hidden among user-block controls at the bottom of the sidebar

• After: Notifications is a top-level sidebar link (same row styling as Challenges/Games/Discussions) with an unread-count badge

• Verified in a real Chromium browser (Playwright) at 375/768/1440px across ~24 pages (home, mocks, challenges, discussions, categories, posts, admin, notifications, users, profile, edit profile) — no horizontal overflow, no JS console errors

Merge Danger

Door: Two-way — purely visual/CSS and helper-guard changes; easy to revert with git revert.

Blast Radius: App-wide (every page touches the shared Tailwind stylesheet and sidebar partial), but changes are additive/cosmetic rather than structural — no schema changes beyond the already-separate Mock feature migration, no removed routes.