feat(paths): per-profile directory resolver

Task 5 of the slice plan. resolveProfilePaths(profile='default')
returns { profileDir, dbFile, mediaDir } under
{userData}/Inkling/profiles/<profile>/ and ensures mediaDir exists
via recursive mkdirSync. Slice scope is single profile 'default'
(spec §1.1, §2.4).

Verification: `npm run typecheck` exits 0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/main/paths.ts

@@ -0,0 +1,20 @@
+import { app } from 'electron';
+import { join } from 'node:path';
+import { mkdirSync } from 'node:fs';
+
+export interface ProfilePaths {
+  profileDir: string;
+  dbFile: string;
+  mediaDir: string;
+}
+
+export function resolveProfilePaths(profile = 'default'): ProfilePaths {
+  const root = join(app.getPath('userData'), 'Inkling', 'profiles', profile);
+  const p: ProfilePaths = {
+    profileDir: root,
+    dbFile: join(root, 'inkling.sqlite'),
+    mediaDir: join(root, 'media')
+  };
+  mkdirSync(p.mediaDir, { recursive: true });
+  return p;
+}