Move LoRA management into generation panels

[brxs]

Summary

• move LoRA installation, management, selection, and strength steering into one contextual control on every generation surface
• replace the always-visible rack with a stable-height trigger and anchored panel
• make Off, On, Bypassed, incompatible, unavailable, and stack-full states explicit in text and accessibility attributes
• centralize adapter registry/install lifecycle state while keeping each generation form's pending LoRA stack independent
• update the issue Build: import & manage Stable Audio 3 LoRA finetunes (gated on #44 / ADR-0028) #66 implementation plan, hardware/UX checklist, and regression coverage

Why

PR #91 splits one workflow between Settings and the generation panels, while the rack grows with the installed adapter library and makes active versus bypassed state difficult to read. This proposal keeps the generation panels compact and puts steering and lifecycle actions in the context where they affect the next generation.

The /api/generate LoRA payload, backend validation, registry/import behavior, SA3 pin, and merge-at-load runtime are unchanged.

Validation

• just check
• 209 backend tests passed
• 593 frontend tests passed
• 333 Rust tests passed; 1 timing diagnostic ignored
• lint, formatting, TypeScript, and Clippy passed

Manual follow-up

The remaining native install, listening, long-name, dense-library, and narrow-layout checks are tracked in docs/issue-66-checklist.md and .plans/issue-66-lora-generation-ui-redesign.md.

Generated-audio clipping is intentionally not patched in this proposal. The upstream serializer behavior is tracked in Stability-AI/stable-audio-3#78.