Add public Forums sub-app and shared forum editing

[jmgasper]

Companion API: topcoder-platform/forums-api-v6#16

Adds the Forums sub-app at /forums and on the forums subdomain, with the supplied Figma category index, topic list, thread view, sidebar, typography, colors and SVG assets. My Drafts is omitted. Guests can browse and return to the same page after login; members can create threads, reply, react and watch accessible discussions.

Public pages reuse the Opportunities forum components and services for rated handles, @mentions, nested replies, quoting, editing, Markdown previews and moderation. The shared editor gains the remaining Figma toolbar controls and configured Filestack attachment uploads. Identity-dependent caches are keyed by member/roles, and the API remains authoritative for restricted categories and all mutations.

Validation:

• yarn lint and optimized builds for both dev and prod pass (8 GiB Node heap).
• 58 focused tests pass, covering the shared forum workflows, guest interaction, rendered index/category/thread routes through the actual platform router on both host types, attachments, rated handles and mentions.
• Chrome QA against the migrated dev API covers desktop/mobile category index, disclosure, category pagination, search, thread detail, guest login with preserved return URL, signed-in creation/reply composers, live member autocomplete, mention insertion, formatting and Markdown preview. No test messages were posted to migrated discussions.
• Figma comparison corrected routing, control box sizing, shared title/button typography, paragraph spacing, post badges/dividers and mobile headings/sidebar layout. Measured desktop titles are Figtree 32/40 px, topic titles 20/30 px bold, and mobile headings 28/36 px. Visible forum icons load at their supplied 20/24 px dimensions; mobile views have no horizontal overflow. The existing externally rendered universal navigation/footer are reused; their current branding differs from the static Figma mock.
• The real Filestack smoke test passed after browser file access was enabled: the approved generated text file uploaded, its HTTPS Markdown link was inserted, and the preview rendered the attachment link. The unposted test draft was cleared. Attachment success/failure/size validation is also covered by focused automated tests.

Deploy the companion forums-api-v6 public-forums change first. Hosting must route the forums hostname and deep links to Platform UI. Setup and verification notes are in src/apps/forums/README.md. Production migration/cutover remains gated on dev acceptance.