Preserve read-only browsing on older platforms

[hamzamerzic]

Summary

• fall back to read-only discovery of flat and directory-shaped skills when the v2 Skills route is unavailable
• label compatibility mode clearly and keep catalog installs and removals unavailable there
• render skill content as safe plain text on the older runtime generation that cannot load the current Markdown renderer
• preserve the complete v2 experience whenever the Skills API is present

Why

This is a follow-up to #4. The v2 app and platform support were released as a pair, but an app update can still reach an owner before their platform update. A missing route currently makes the whole app fail even though the skill files remain readable through shared storage.

Platform PR #178 fixes Markdown imports for API-capable current platforms. This change is complementary: it preserves browsing on platforms old enough to lack /api/skills, where the newer compiler mapping is not available.

Testing

• MOBIUS_FRONTEND_NODE_MODULES=/data/platform/frontend/node_modules npm test (98 passed)
• app validation (0 errors; 3 existing static external-host warnings)
• git diff --check