v1.9.1: fix Runtime switching and adopt verified external runtimes

[weidonglang]

Scope

Implements and closes #141 while keeping #130 open for broader Runtime lifecycle work.

• fixes managed Runtime switching that could appear to do nothing
• adds durable row feedback, visible plan/error/result state, focus, stale-request guards, and duplicate-click prevention
• resolves switch targets from backend-trusted stable Runtime IDs and allowlisted modes
• adopts strongly verified external JDK, Python, Node.js, Go, Maven, and Gradle installations without modifying external directories
• adds provider-aware nvm/fnm/Volta/Scoop/rustup switching and project-scoped .NET global.json selection
• binds plans to source authority, environment and file/provider fingerprints, expiry, backups, confirmation tokens, single consumption, post-verification, rollback, and persistent recovery
• restores saved Runtime switch backups on initial page load after application restart
• removes the public legacy direct switch_runtime command

Final validation

• Rust: 230 passed, 0 failed
• Clippy: passed with -D warnings
• Frontend build: 113 modules
• Frontend acceptance: 268 selectors
• Visual acceptance: 30 passed
• Feature acceptance: 300 total; 254 passed, 0 failed, 40 policy skips, 6 manual/policy
• P0/P1 failures, backend-only, ui-only, missing, partial, deferred, and toast-only: 0
• GitHub Actions for source commit 4137fbe: passed

Exact-head Windows ReleaseLab

• final NSIS/MSI hashes matched after guest transfer
• managed Node switch and restore passed
• strongly verified external Node adoption and restore passed
• external directory file hashes remained unchanged
• persisted recovery backups were visible after application restart
• MSI install/start/uninstall passed
• v1.9.0 to v1.9.1 upgrade preserved exact settings and Profile hashes
• final process and uninstall-registration counts were zero

Reviewed publication assets

• NSIS: DevEnv.Manager_1.9.1_x64-setup.exe, 2,951,926 bytes, ce4b9020d47258dd1668f0237a0b3fd0b061ddfc20c938fed56f5ddcf5cd0352
• MSI: DevEnv.Manager_1.9.1_x64_en-US.msi, 4,943,872 bytes, cf26c80e2f67eb61b3c000e8bb91dd30af96deb6131268ec8350819e85a540c3

Full evidence and non-blocking disclosures are recorded in docs/release-v1.9.1.md.

#130 and #133 remain open because they contain follow-up work outside this hotfix.

Closes #141

[weidonglang]

Reopening immediately to retrigger GitHub Actions for the latest source-frozen head; no scope or review state change.