feat(workspaces): bulk-clear unmapped folders + clearer approval opt-out - #172

What & why

Two small UX gaps surfaced while using the Workspaces tab and the meta-tool approval prompt.

1. Clear all unmapped folders at once

The Workspaces tab lists folders connected clients reported but that aren't mapped to a FeatureSet yet (amber Unmapped cards). You could filter to them, but there was no way to dismiss them in bulk — and dismissing them is exactly how you get McpMux to re-offer the "map this folder?" prompt.

• SessionRootsRegistry::forget_unmapped_roots() drops the unmapped roots from the in-memory session-roots registry. A session left with no roots is removed entirely (along with its resolution snapshot + probe throttle) so its next tools/list re-probes the peer and the resolver re-fires WorkspaceNeedsBinding — i.e. the prompt appears again. Sessions still holding a mapped root are left untouched.
• clear_unmapped_reported_roots Tauri command snapshots the bound roots (same exact-match rule the UI uses to label a card "Unmapped"), clears the rest, and emits SessionRootsChanged so the tab refreshes.
• The Workspaces tab gains a Clear unmapped button (shown only while there are unmapped folders) behind a confirm, with a toast that sets the "you'll be asked again" expectation.

2. Make the approval opt-out noticeable

The "manage approval prompts" escape hatch on the meta-tool approval dialog was an 11px muted link that was easy to miss. It's now a bordered, icon-led button above a divider so the path to turning these prompts off is actually visible. Behavior is unchanged (deny the current request, then route to the Built-in tab where the toggle lives).

Tests

• Rust unit tests for forget_unmapped_roots — clears unmapped sessions (and resets their snapshot so the next resolve re-prompts), keeps mixed sessions that still hold a mapped root.
• New WorkspacesClearUnmapped.test.tsx — the button only appears while there are unmapped folders, and confirming it calls clearUnmappedReportedRoots.
• The existing approval-dialog test stays green (testid-based; only the link's styling/label changed).

Validation (local)

cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, pnpm typecheck, pnpm -r lint (0 errors), Rust unit tests, and the full vitest suite (200 passed) all pass.