feat(ui): image viewer with zoom, pan and gesture support

[Tryanks]

Summary

The image lightbox was a 1200px dialog card with a static img inside. It is now an ImageViewer entity on a host-filling dialog that zooms and pans, replacing attachments::open_image_lightbox for the composer strip, sent-message thumbnails, Markdown images and image-link badges.

 Overlays (dialog stack)
   Dialog
-    card: title + img().max_w_full().max_h(…)
+    fill(): absolute inset_0, surface brings its own chrome
+      <ImageViewer> (crates/ui/src/image_viewer.rs)
+        stage  — img placed at fit × zoom + offset, clamped to the stage
+        toolbar — title · zoom out · % · zoom in · fit ⇄ actual size · close

Gestures, all resolved against the last drawn frame:

wheel (lines)            zoom about the pointer, one notch = ×1.25
wheel (pixels / touch)   pan; with ctrl/⌘ held: zoom (browser convention)
pinch (trackpad, DM, Wayland/X11, native mobile) zoom about the pinch centre
drag / touch pan         pan while the image exceeds the stage
double-click / tap       fit ⇄ actual size (or 2× fit when that is nearer)
= + ⌘= / - ⌘- / 0 ⌘0 / 1 ⌘1   zoom in / out / fit / actual size (glide 200 ms)
click beside the image, Esc, ×   close

Placement is clamped so the image is centred on any axis where it is smaller than the stage and never leaves a gap where it is larger; small images are not enlarged at fit, and 1:1 is always reachable. Keys, buttons and double-click glide via gpui_base::motion::transition; pointer gestures track directly.

Survey before building: neither gpui-base 0.7 nor gpui-kit ships an image viewer, lightbox or zoom/pan primitive (only Carousel and Attachment thumbnails), so this composes gpui's PinchEvent/ScrollWheelEvent/click-count with gpui-base Dialog and transition.

Also in this PR:

• Dialog::fill() on Tcode's dialog wrapper, the one addition needed to host a full-window surface on the existing stack.
• window_caption::traffic_light_inset(window) is now the owner of the macOS traffic-light inset; the shell's nav bar uses it instead of its private constant, and the viewer toolbar uses it too.
• Four Lucide icons (zoom-in, zoom-out, maximize-2, minimize-2) bundled in assets/icons, since the bundled component icon set does not contain them (they rendered blank on first run).

Evidence

• Before: fixed-size card, no zoom or pan; a large screenshot was only ever shown shrunk.
  After: launched target/debug/tcode on a throwaway TCODE_DATA_DIR, pasted a 2400×1600 PNG into the composer and opened it from the thumbnail. Verified: fit at 49%, toolbar clear of the traffic lights, = key → 61% and fit button appears, drag pans the enlarged image, window at 480px wide re-fits to 27% with the compact toolbar, dark mode renders the dark scrim and opaque toolbar. Double-click could not be driven by the automation tool (it posts two single clicks); that path is covered by the headless test.

• Headless UI tests in image_viewer::tests drive the window the way the app does (simulate_event for wheel and pinch, mouse down/move/up, keystrokes, clock advance for the glide):

pointer_gestures_zoom_about_the_pointer_pan_within_the_stage_and_close
  fit 400×200 → wheel notch at centre → 500×250 centred
  pinch ×2 at the corner → 1000×500, held flush to the left edge
  drag 100px → origin -100 ; trackpad pan -50 → origin -150
  click beside the image → viewer gone
keys_glide_between_fit_and_actual_size_and_escape_closes
  "1" → still fitted on the first frame, actual size after 200 ms
  "0" → fitted again ; escape → viewer gone

• Checks run locally on this branch: cargo fmt --all --check, cargo clippy --workspace --all-targets --locked -- -D warnings, cargo nextest run --workspace --locked (892 passed), cargo machete. Not run here: Windows, Linux, iOS, Android and Web builds (CI).

Merge Danger

Door: two-way

Revert restores the previous card; no persisted state or protocol is touched.

Blast Radius: presentation

Every image-open path in the UI now lands in the viewer. The dialog wrapper gains an opt-in fill() and existing dialogs are laid out exactly as before. The shell nav bar's traffic-light inset is computed by the shared helper with the same values. Pinch on iOS/Android depends on the platform declaring a native pinch recogniser; gpui's portable recogniser does not implement pinch yet, so touch there gets tap, double-tap and pan.