feat: add isolated test packages and dev profiles

[grazzolini]

Summary

• Add isolated macOS, Android, and Flatpak test packages with identities derived from the production configuration.
• Default pnpm dev to test file storage, with --production-data for explicit production access and normal development tool-cache locations preserved.

Motivation

Allow test packages to coexist with production and exercise packaging changes without sharing production application state.

Closes #547

Changes

• Centralize production storage defaults across Python, Rust, and packaging scripts; derive test data, model-cache, and sync paths from them.
• Add TuneForge Test labels, marked assets, an in-app TEST indicator, and actual package-ID diagnostics. Keep development WebView identity, origin, and browser settings unchanged in both dev modes.
• Start development with an owned loopback backend, verify its health URL and data root, and clean up only launcher-owned processes.
• Restore temporary Android resources after builds and separate Flatpak test manifests, refs, build outputs, and repositories. Document test packaging workflows.

Testing

• pnpm lint, pnpm typecheck, pnpm test, and cd apps/desktop/src-tauri && cargo check passed during implementation. The full suite passed before the final launcher-only follow-ups.
• node --test scripts/dev.test.mjs: 17 tests passed after the final launcher changes, covering cache preservation, profile isolation, readiness, socket ownership, reload, and shutdown.
• pnpm package:mac:app -- --test: built successfully; verified bundle ID, executable, icon, and bundled storage configuration. Local macOS artifacts remain unsigned/not notarized as documented.
• pnpm package:android: built successfully; APK identity, label, signer, JNI checks, and generated-resource restoration verified. Manual testing confirmed debug and optimized local builds use the test identity and in-app visuals.
• Manual Linux checks passed for both development modes. Production Flatpak packaging completed and its installed icon was confirmed unchanged.
• Synthetic visual QA passed for TEST branding across full, compact, and mobile layouts. Correctness and contract reviews completed.

Remaining validation and limitations

• Draft while the Linux test Flatpak build and installed-package checks are in progress. Command: pnpm package:linux:flatpak -- --test.
• Android adaptive masks can hide the corner marker on the test launcher icon. The TuneForge Test label and in-app TEST indicator distinguish these builds; Android-specific artwork is deferred.
• The complete install/update/uninstall isolation matrix and Release Package Gate evidence remain incomplete. No release artifacts are being published by this PR.
• Flatpak cache reuse improvements remain tracked separately in Investigate and improve Flatpak cache reuse across releases and source changes #595.

Checklist

• Conventional Commits title
• Lint, typecheck, and full tests passed; final launcher changes have focused regression coverage
• Changelog and relevant documentation updated
• No HTTP route/schema changes; contract regeneration not required
• No new runtime dependencies, model-weight policy changes, generated package artifacts, or caches committed
• No secrets, credentials, or copyrighted audio added
• Complete platform package-launch and isolation evidence from the Release Package Gate