Cobalt liked-by and reposted-by lists

The post menu showed a post's counts as text but never who was behind them (issue #104). This adds "Liked by (N)" and "Reposted by (N)" entries to the post menu, opening the same avatar-row list screen the followers/following screens already use.

Changes

• src/app/graph.{c,h} — two new cobalt_graph_kinds (LIKES, REPOSTED) wired into the existing list screen: list_for, begin_fetch, titles ("Liked by" / "Reposted by"), empty messages, and cobalt_graph_view_open_likes, which mirrors the follows open (rewind, record the post URI, fetch fresh). cobalt_graph_kind_is_follows covers them, so A opens a profile like every other row-list. begin_fetch now takes the view rather than the kind, since likes/reposts page against the URI the view remembers.
• src/atproto/session.{c,h} — COBALT_JOB_LIKES / COBALT_JOB_REPOSTED_BY, cobalt_session_begin_likes / begin_reposted_by / likes_list. The two kinds share one cobalt_actor_list (s.likes): only one is on screen at a time and every open resets it, so there is no cross-talk. getRepostedBy rides the existing run_actor_list with a fetch_fn; getLikes does not — Wolfram returns wf_agent_actor_like_list, whose rows carry the actor one level down, so it gets its own run_likes with the same locking, reset and paging shape.
• src/app/app.c — COBALT_POPUP_LIKES / COBALT_POPUP_REPOSTS entries in the post menu (with the like/repost icons and the count in the label), a COBALT_SCREEN_LIKES_LIST case in update and draw, and a likes_return so B goes back to the timeline or thread the menu was opened from.
• Tests: test_likes_lists covers the begin guards, the graph open (kind, actor, rewind), the wrong-kind refusal, and the is_follows classification. Host suite: 488 checks, 0 failures. Wii U cross-build clean.

Commit be33e37, pushed. Issue #104 closed.