fix(plugins): unify plugin data directory under tabularis

[debba]

Summary

Plugins were resolved with ProjectDirs::from("com", "debba", "tabularis"), while the rest of the app data (config, themes, AI activity, heartbeat) uses ProjectDirs::from("", "", "tabularis") and the Tauri identifier is tabularis. On macOS and Windows this stored and loaded plugins from a separate com.debba.tabularis directory that also contradicted the documented path, so a plugin placed at the documented location was never discovered.

This unifies plugin storage under the same tabularis directory as all other app data, with no code duplication, plus a one-time migration of any existing plugins.

Changes

• paths.rs — extract shared project_dirs() + a pure, unit-testable unnested_app_dir() helper; add get_app_data_dir(). get_app_config_dir() is refactored onto the same helpers with identical output.
• plugins/installer.rs — get_plugins_dir() now resolves via paths::get_app_data_dir(); add one-time migrate_legacy_plugins_dir() and a pure migrate_plugins_between() core.
• plugins/manager.rs — reuse installer::get_plugins_dir() instead of a duplicate ProjectDirs call.
• lib.rs — run the migration once at startup, before loading plugins.
• Tests — new paths_tests.rs and migration cases in plugins/tests.rs.

Resulting plugin directory

OS	Before	After (matches docs)
Linux	~/.local/share/tabularis/plugins/	~/.local/share/tabularis/plugins/ (unchanged)
macOS	~/Library/Application Support/com.debba.tabularis/plugins/	~/Library/Application Support/tabularis/plugins/
Windows	%APPDATA%\debba\tabularis\data\plugins\	%APPDATA%\tabularis\plugins\

The documented paths (plugins/PLUGIN_GUIDE.md, plugins/README.md, plugins/PLUGIN_TUTORIAL.md, scaffolder justfile.tmpl) already declared tabularis, so no doc changes are needed — the code now matches them.

Migration

migrate_legacy_plugins_dir() runs once at startup and is idempotent. It moves plugin folders from the legacy com.debba.tabularis location into the new one only when the target is still empty (never clobbering an existing install), is a no-op on Linux (the two paths are already identical), and a no-op when there is nothing to migrate.

Testing

• cargo test — paths_tests (4) and plugins::tests (6, including 4 new migration cases) pass.
• cargo check --tests — clean.

Closes #257

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (6 files)

• src-tauri/src/lib.rs — startup migration call ordering is correct
• src-tauri/src/paths.rs — clean extraction of shared helpers, consistent fallback paths
• src-tauri/src/paths_tests.rs — good coverage for unnested_app_dir and directory name assertions
• src-tauri/src/plugins/installer.rs — safe idempotent migration logic with proper guards
• src-tauri/src/plugins/manager.rs — correctly deduplicates ProjectDirs resolution
• src-tauri/src/plugins/tests.rs — comprehensive migration test cases including edge cases

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_{Reviewed by kimi-k2.6-20260420 · 107,810 tokens}