Cobalt full-size image viewer

Images only ever rendered as thumbnails inside the card — a fraction of the TV's 720p, with no way to see the whole picture (issue #100). This adds a full-size viewer, opened from the post menu.

Changes

• src/ui/imageview.{c,h} — new cobalt_imageview overlay. It opens on a post, holds copies of that post's image list (a background refresh cannot rewrite the pictures mid-view), and draws the focused picture contain-fitted to the whole surface, centred. Alt text sits beneath (up to 3 lines), "N of M" top-right, Left/Right cycles multi-image posts, and B, A or a tap closes. cobalt_imageview_update consumes the frame whenever the viewer is open, so the screen underneath never acts on a press the viewer already saw.
• src/ui/imagecache.{c,h} — cobalt_imagecache_create_sized(max_dimension, fit, entries, loaders) takes slot and loader counts at creation where cobalt_imagecache_create fixed them at compile time. The viewer's caches are two slots and one loader each — a person looks at one picture at a time — decoded CONTAIN at the surface's own height (720 TV, 480 GamePad), where a card thumbnail stays capped at 320.
• src/ui/render.{c,h} — cobalt_render_set_viewer() / cobalt_render_viewer(): a third borrowed cache pointer per surface, alongside the avatar and thumb ones.
• src/app/app.c — COBALT_POPUP_IMAGE ("View image", with the image icon) joins the post menu when the selected post carries pictures; popup_choose re-derives the post from the thread or timeline selection and opens the viewer. The update gate checks the viewer before the popup; the draw path draws the overlay instead of the screen.
• src/main.c — tv_viewer / drc_viewer caches, created, pumped and destroyed alongside the others.
• Tests: test_imageview covers open on a bare post (no-op), copy semantics, cycle-with-wrap both directions, close clearing state, and the single-image no-cycle case. Host suite: 476 checks, 0 failures. Wii U cross-build links clean (needed a Wolfram build-wiiu rebuild for the OAuth node's wf_agent_set_bearer).

Opened from the post menu rather than direct tap-on-thumbnail: the card layout has no per-cell rects to hit-test today. That stays a follow-up.

Commit 3afe065, pushed. Issue #100 closed.