feat: surface per-module load errors in Settings → Modules

[Teamingzooper]

Summary

When a directory under `modules/` or `~/.nexus/modules/` can't be loaded as a module (usually a malformed `nexus-module.json`), Nexus previously just logged a warning and silently skipped it. Users would notice "Hey, my custom module isn't showing up" with no signal about why.

This PR surfaces those load errors directly in Settings → Modules, with the validation issue paths spelled out where possible.

Branch base: `feat/settings-import-export` — fourth in the stack. Merge in order: #6 → #7 → #8 → this.

What's shown

For each rejected module directory:

• Its absolute filesystem path (monospace, so it's copy-pasteable).
• A human-readable error message.
• For Zod manifest-validation failures: a bulleted list of each issue, formatted as `path.to.field: explanation` (e.g. `url: module url must be https`).
• For JSON parse errors: the parser's own message (`Manifest is not valid JSON: Unexpected token …`).

The panel only renders when there are errors, so the UI stays clean for users with no broken modules. Clicking the existing Reload modules button re-runs the scan and refreshes both the working modules list AND the errors list — fix-and-reload loop converges on an empty panel.

Scope (deliberately narrow)

The original ask included three "module devtools" capabilities:

1. Better error surfacing when a manifest is malformed ← shipped here.
2. "Validate manifest" command — overlapping with Native OS notifications: [Nexus] <instance> — <message> #1 (showing manifest errors). A standalone file-picker validator for module authors is genuinely useful but doesn't fit a 1-PR scope. Punted to v2.
3. Hot-reload local modules without restarting — the existing Reload modules button is the manual-trigger version. A file watcher would be nicer but adds a dependency (chokidar/native FS watch) and dev-only UX considerations. Punted to v2.

Implementation

• New `ModuleLoadError` type in shared types.
• New IPC channel `nexus:modules:list-errors`.
• `ModuleRegistryService` clears its error list at the start of each load and pushes a structured `{ path, message, details? }` for each rejected module. Errors are categorised by type: Zod (paths + issue messages), SyntaxError (JSON parse), or generic Error.
• Renderer store fetches errors in `init()` alongside modules, and in `reloadModules()` both lists refresh in parallel.
• Settings → Modules renders the panel conditionally when `moduleErrors.length > 0`.

Test plan

• `npm run typecheck` — clean
• `npm run test` — 178/178 passing
• `npm run build` — renderer + main both clean
• Manual smoke: drop a directory under `~/.nexus/modules/` with a broken `nexus-module.json` (e.g. `{ "id": "test" }` — missing required `name` / `version` / `url`). Click Reload modules. The error panel should appear listing the missing fields with their schema paths.
• Manual smoke: drop a directory with malformed JSON (`{ this is not json`). Reload. The panel should show the JSON parse error.
• Manual smoke: fix one of the broken modules, leave another broken, reload. The error panel should now show only the still-broken one.

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