feat(studio): workspace settings overlay

[pbertsch]

Summary

Adds a ⚙ toolbar button that opens a form for the four probe.yaml fields users actually touch. Saves write back to `probe.yaml` in the active workspace, preserving all other keys.

v0.7.1 PR 2 of 3.

Fields exposed

Form label	YAML path	Why this one
Agent port	`agent.port`	Default 48686; users only change for parallel runs or port conflicts
Default step timeout	`defaults.timeout`	Slow CI environments / heavy animations need longer than 30s
Preferred iOS UDID	`device.ios_device_id`	Avoid `--device` on every run when you have multiple sims
Preferred Android serial	`device.android_device_id`	Same as above for emulators

Deliberately tight — extending the field set is cheap once the plumbing exists, but landing a small form first beats arguing about which 20 fields to expose.

Implementation notes

• Preserve unknown keys on save. `SaveWorkspaceSettings` reads the existing file as a raw `map[string]any` rather than as a `Config` struct, mutates only the four managed keys, and writes back. So a power user with `recipes_folder`, `cloud`, `plugins`, etc. keeps everything they had.
• Empty fields are dropped, not zero-written. Clearing a field in the UI removes the key from `probe.yaml` (and removes the parent map if it goes empty), maintaining the convention that `probe.yaml` only lists fields the user explicitly set.
• Validation runs before disk touch. `agent.port` is range-checked (0–65535); `defaults.timeout` is parsed as a Go duration. Invalid input → clear error, no half-written file.
• No-workspace state. If the user opens settings before picking a workspace, the form is hidden and a hint shows to use 📂 first.

Test plan

• `go build ./...` clean
• `go test ./...` — full suite green
• `cd studio && go vet ./...` clean
• `tsc --noEmit` clean
• Manual: empty workspace — settings on a workspace with no `probe.yaml`, save → file created with only the fields filled in.
• Manual: existing workspace — settings on a workspace with rich `probe.yaml` (e.g. `tests/recipes`, custom `reports_folder`, etc.), save → those keys still present, only the four managed fields updated.
• Manual: invalid input — port `70000` or timeout `5 weeks` → save fails with toast, no file write.