fix(storage): never fall back to launch-directory database

[ElbertePlinio]

Problem

src-tauri/src/lib.rs falls back to relative pickforge.db when pickforge_home(None) fails. A release-binary smoke launched from an environment without a usable home directory created pickforge.db, pickforge.db-shm, and pickforge.db-wal in the release worktree root.

Normal installed/isolated launches with HOME set still resolve to ~/.pickforge/pickforge.db, so this is not the current v0.1.10 release gate. The fallback still violates the workspace storage rule: a process launch directory must never silently become its data directory.

Acceptance

• Remove the relative launch-directory fallback
• Resolve to a documented safe user-data location or fail with an actionable startup error
• Preserve explicit automation/config overrides
• Add path-resolution coverage for missing/unusable home state
• Smoke normal startup and prove the source tree stays clean

Evidence

• Source: src-tauri/src/lib.rs:37-40
• Reproduction: release binary launched with no usable home context created the three SQLite files in the worktree root
• Files were removed after capture; no user data was involved