fix(paths): support HPC symlinks + drop home/project gate for read-only ops

[RealZST]

Summary

Custom config paths (preview, add) and several desktop-side read/UX operations were rejecting valid paths in two scenarios:

1. Symlinked $HOME (e.g. Lumi's /users/<user> → /pfs/lustrep*/users/<user>): is_path_allowed compared canonical file paths against the symlink-form home, never matching the prefix.
2. Work outside $HOME (HPC scratch dirs, external volumes, network shares): the home/project gate forced users to register a project just to track a single config file — unnecessary friction since the OS already gates reads via filesystem permissions.

Backend

• is_path_allowed: canonicalize both sides before prefix comparison; reject non-existent paths (mirrors desktop semantics). Remains in use only as the deploy/write gate.
• resolve_and_validate_config_path: extracted to hk_core::sanitize as the single authority for both web and desktop. Drops the home/project gate; keeps .. rejection, exists() check, and home-itself rejection (now via canonicalize so trailing slashes / symlinks all reject correctly).
• Desktop is_path_within_allowed_dirs: deleted entirely (was the equivalent gate for read-only ops; same trust-model justification).

Frontend

• humanizeError accepts unknown (Tauri IPC rejects with reified HkError objects, not strings — String() wrapping produced "[object Object]").
• Preserve specific message for PathNotAllowed / PermissionDenied (previous git-flavored "repository may be private" wording was misleading for fs-perm errors).
• IME composition guard with e.keyCode !== 229 fallback (macOS WebKit isComposing is unreliable).

Test plan

• 365 unit + integration tests pass (`cargo test --workspace` + `npx vitest run`)
• clippy clean across workspace (`cargo clippy --workspace -- -D warnings`)
• tsc + biome clean
• End-to-end on Lumi (web mode): opening `.claude.json` preview no longer shows "Access denied"; adding paths under `/pfs/lustrep3/scratch/` works without registering a project
• End-to-end on macOS desktop (`cargo tauri dev`): D behavior verified for `/etc/hosts`, `/usr/share/dict/words`, `/etc/master.passwd`; error toasts now show real cause; IME confirm-on-Enter no longer triggers form submit; `/etc/../etc/hosts` and `~` show specific errors

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