fix(nodes): pre-validate storage_dir + unwrap daemon error envelope

[Nic-dorman]

Summary

Customer reports of `Failed to add nodes: {"error":"I/O error: Access is denied. (os error 5)"}` on Windows after v0.6.7 (the first build where the user-chosen `storage_dir` is actually honored at node-add time). Two problems in one report: the path is unwritable to the daemon, and the toast surfaces the raw JSON envelope verbatim.

Three defences:

1. `probe_writable` Tauri command (`src-tauri/src/lib.rs`) — writes a sentinel, fsyncs, deletes. Returns the raw OS message + path on failure.
2. Settings picker gates on the probe. `pages/settings.vue::pickStorageDir` calls `settingsStore.probeStorageDir(path)` before `setStorageDir`. On failure the path is not saved and the error is shown inline under the card and as a global banner in `layouts/default.vue`. Success toast changes from "Storage directory updated" to "Storage directory verified".
3. Startup re-probe. `app.vue` calls `settingsStore.revalidateStorageDir()` after `loadConfig` so a previously-working path that's gone offline is caught before the user reaches Add Node. Non-blocking; does not clear the saved path.
4. Envelope unwrap in `utils/daemon-api.ts::request`. Parses `{"error": "..."}` out of 4xx/5xx bodies before throwing. Falls back to the raw body when JSON parses but lacks `.error`, or when the body isn't JSON at all (network / Tauri errors). Covers every daemon error toast.

Common failure categories this catches: volume read-only flag (USB / VHDX / BitLocker To Go), second internal drive with default ACL, OneDrive Files-On-Demand placeholders, restrictive network-share ACL.

Test plan

• CI vitest (local Win + Node 20.18 hits the known `@vue/test-utils` dev-env issue)
• Plug in any USB; `diskpart > attributes volume set readonly`. In Settings → Storage Directory pick the USB root. Expect inline error + global banner; path not persisted.
• Pick a writable folder (e.g. `C:\Users<me>\storage`). Expect "Storage directory verified" toast, banner clears, Add Node succeeds.
• With a saved unwritable path, eject the USB and relaunch the app. Banner should appear on startup before the user opens Settings.
• On Linux/macOS: pick a folder with `chmod -w`. Expect the same inline-error path.
• Trigger any other daemon error (rewards address validation, network port collision) — toast should show the unwrapped daemon message, not a JSON envelope.

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