Treat unreadable `project.json` files as corrupt entries instead of aborting inventory

[kwannoel]

Summary

Evidence:

• src-tauri/src/storage.rs:152-175 records JSON parse failures in corrupt_entries, but it still does fs::read_to_string(&project_file)?; a single unreadable file aborts the entire listing.
• restore_sessions() depends on list_projects() at src-tauri/src/commands.rs:336-348, so one unreadable project can block startup session restore.
• MaintainerScheduler::start() also depends on list_projects() at src-tauri/src/maintainer.rs:149-160, so the same error can suppress background maintainer runs for otherwise healthy projects.
• Existing storage coverage only checks malformed JSON (src-tauri/src/storage.rs:403-421), not unreadable files.

Remediation:

• Handle read_to_string failures the same way as parse failures: record a CorruptProjectEntry and continue.
• Add a regression test for unreadable project.json.

Maintainer Metadata

• Fingerprint: commands|corruption|failure|filesystem|inventory|listing|startup|storage
• Symptom Type: startup listing failure
• Affected Files: src-tauri/src/storage.rs, src-tauri/src/commands.rs, src-tauri/src/maintainer.rs
• Keywords: storage, project-inventory, filesystem, corruption