feat(pinchflat): Media Profiles — named download presets per Library source

A MediaProfile bundles { kind, quality, options, outputTemplate, extraArgs }
that a Source points at via Source.profileId, overriding the global defaults
field-by-field for that source's downloads.

Backend mirrors CommandTemplate: profiles.ts (createJsonStore, PROFILES_MAX,
isProfileLike + sanitize), CRUD IPC (profiles:list/save/remove) + preload +
useProfiles store; Source.profileId + sources:set-profile. Pure resolution
in @shared/profileResolve.ts (profile → global, per field; unit-tested).
Applied in the sources-store enqueueItems; a per-download collectionOutput-
Template override threads to buildArgs so a profile's outputTemplate wins.

UI: ProfileManager (mirrors TemplateManager) in a new Settings → Advanced →
Media profiles card, plus a per-source Profile picker in the Library source
detail. Both live-verified in the preview; 354 tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/core/validation.ts

@@ -7,7 +7,14 @@
  */
 
 import { isAbsolute } from 'path'
-import type { HistoryEntry, ErrorLogEntry, CommandTemplate, Source, MediaItem } from '@shared/ipc'
+import type {
+  HistoryEntry,
+  ErrorLogEntry,
+  CommandTemplate,
+  MediaProfile,
+  Source,
+  MediaItem
+} from '@shared/ipc'
 
 // --- Path-traversal sanitization (audit S4) ---------------------------------
 
@@ -82,6 +89,17 @@ export function isTemplateLike(o: unknown): o is CommandTemplate {
   return typeof t.id === 'string' || typeof t.id === 'number'
 }
 
+/**
+ * A persisted profiles.json row must at least be an object carrying an id; the
+ * rest is coerced by profiles.ts's sanitize(), so a weak shape check plus
+ * normalisation there is enough (same approach as isTemplateLike).
+ */
+export function isProfileLike(o: unknown): o is MediaProfile {
+  if (!o || typeof o !== 'object') return false
+  const p = o as Record<string, unknown>
+  return typeof p.id === 'string' || typeof p.id === 'number'
+}
+
 /** A persisted sources.json row must have the right shape or it's dropped on read. */
 export function isValidSource(o: unknown): o is Source {
   if (!o || typeof o !== 'object') return false