fix(app): fall back to default data folder when persisted path is stale

[santhiprakash]

Describe Your Changes

• Fix resolve_jan_data_folder() (CLI/headless path) and get_app_configurations() (Tauri app path) so a persisted data_folder that no longer exists or is unreadable falls back to the default Jan data folder instead of returning the stale path.
• Add is_readable_data_folder() and default_data_folder_path_internal() helpers in src-tauri/src/core/app/commands.rs.
• Add regression tests covering both the CLI path (resolve_jan_data_folder) and the app path (get_app_configurations).
• Keep the JAN_DATA_FOLDER environment override as an explicit opt-in that is not validated or fallen back from.

Fixes Issues

• Closes bug: App crash after home folder name change #8794

Self Checklist

• Added regression tests for stale data_folder fallback
• Ran cargo fmt -- src/core/app/commands.rs and cargo fmt --check -- src/core/app/commands.rs (clean)
• Ran cargo clippy --lib (clean)
• Ran cargo test --lib core::app::commands

Problem

After a macOS user home folder is renamed, settings.json still contains the old absolute data_folder (e.g. /Users/<oldname>/Library/Application Support/Jan/data). Both resolve_jan_data_folder() and get_app_configurations() returned that path directly, so the app started against a non-existent directory and crashed.

Triage

• resolve_jan_data_folder() parsed config.data_folder and returned it with no existence/readability check.
• get_app_configurations() parsed the file and returned the stored data_folder without validating it.

Fix

Both read paths now call is_readable_data_folder() on the persisted data_folder. If it is missing or unreadable, they log a warning and fall back to the default Jan data folder. The JAN_DATA_FOLDER environment override still takes precedence and is not validated.

Verification

Before the fix, a stale absolute data_folder returned the stale path:

thread 'core::app::commands::tests::resolve_jan_data_folder_falls_back_when_stale' panicked at src/core/app/commands.rs:491:9:
assertion `left == right` failed
  left: "/tmp/.../stale_home/Library/Application Support/Jan/data"
 right: "/tmp/.../Jan/data"

After the fix, both new tests and the existing command tests pass:

$ cd src-tauri && cargo test --lib core::app::commands
running 6 tests
test core::app::commands::tests::bundle_identifier_matches_tauri_conf ... ok
test core::app::commands::tests::migration_handles_missing_legacy_files ... ok
test core::app::commands::tests::migration_skips_when_canonical_exists ... ok
test core::app::commands::tests::migration_recovers_legacy_then_removes_stale_copy ... ok
test core::app::commands::tests::resolve_jan_data_folder_falls_back_when_stale ... ok
test core::app::commands::tests::get_app_configurations_falls_back_when_stale ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 664 filtered out

$ cd src-tauri && cargo clippy --lib
    Checking Jan v0.8.4 (/mnt/data/open-source/jan/src-tauri)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 5.76s

Formatter: cargo fmt -- src/core/app/commands.rs followed by cargo fmt --check -- src/core/app/commands.rs returned no diff.

Notes

• This is a runtime fallback; it intentionally does not rewrite settings.json on disk. The optional companion change to persist a tilde-based default path is left for a separate commit/PR as suggested in the issue.
• The new tests take an ENV_TEST_LOCK because they redirect XDG_DATA_HOME / XDG_CONFIG_HOME / HOME (for the CLI test) and APP_NAME (for the Tauri test) while running in parallel with other tests.