feat(mobile): add post creation and comments

Post creation: gallery/camera picker (expo-image-picker) uploads each
image straight to Supabase Storage via a presigned PUT
(posts.getPresignedUrl + expo-file-system), then posts.create with the
resulting storage keys — reachable via a new "+" header button on the
Feed tab. No client-side blurhash (browser Canvas API, no RN
equivalent) or AI-disclosure/mention support yet, matching the rest of
mobile's plain-text captions.

Comments: a dedicated screen (posts/[postId]/comments) for reading,
posting, and deleting one's own comments, reachable by tapping the
comment icon on a PostCard. Ported from web's PostDetailDialog comment
panel, scoped down to just that panel since mobile's image full-view
already has its own route.

mobile/src/api/types.ts

@@ -8,3 +8,4 @@ export type FeedPost = RouterOutputs["feed"]["getFeed"]["posts"][number];
 export type ExplorePet = RouterOutputs["explore"]["listPets"]["pets"][number];
 export type NotificationItem = RouterOutputs["notifications"]["list"]["notifications"][number];
 export type Conversation = RouterOutputs["conversations"]["list"][number];
+export type PostComment = RouterOutputs["comments"]["listByPost"]["comments"][number];