screen.html + docs: migrate canvas-sizing + RFB design prose (#713 batch 3)

Expanded the /screen endpoint description in docs/web-ui.md to
cover the substantive design rationale that lived in screen.html
comments. screen.html shrinks correspondingly.

Moved to docs/web-ui.md::Per-agent endpoints (GET /screen):
- Deliberate thinness — minimal RFB renderer; noVNC vendoring
  path called out for production-grade replacement.
- Fit-mode flex-item min-width:auto clamp: a flex item's automatic
  minimum size resolves to the canvas's intrinsic framebuffer
  resolution and clamps CSS max-* back up, making fit mode a
  silent no-op. The fix (flex: none + min-width: 0 + min-height: 0
  + explicit px sizing via relayoutCanvas()) is now documented.
- localStorage persistence for fit-mode (key screen-fit; default
  on).
- Pointer rescale in sendPointer keeps clicks accurate.
- ExtendedDesktopSize pseudo-encoding (-308 rect) gates the
  match-size button.

Collapsed in screen.html: 8 #NNN cookies scrubbed across all
inline comments. #133 (canvas-sizing fit bug — closed) ×5,
#52 (noVNC vendoring — closed) ×1, #14 (relative URL — closed
my piece) ×1, plus one CSS-block #133. Each comment shrinks to
a brief pointer.

screen.html: 8 → 0 #NNN cookies (100% reduction). Net ~26 lines
of substantive prose moved into docs/web-ui.md.

docs/web-ui.md

@@ -1055,14 +1055,26 @@ shaped).
   `wait_for_login` entry.
 - `GET /events/history` — replay buffer for the terminal.
 - `GET /screen` — VNC viewer page (minimal RFB-over-WebSocket
-  renderer). Only accessible when `hyperhive.gui.enable = true`
-  in the agent's `agent.nix`; the harness shows a 🖥 screen link
-  in the state row when `gui_vnc_port` is present. Toolbar:
-  `⤢ fit` CSS-downscales the canvas to the window; `⤡ match size`
-  sends an RFB `SetDesktopSize` request so the server (weston)
-  changes its real output resolution to the window dimensions —
-  enabled once the server advertises the `ExtendedDesktopSize`
-  pseudo-encoding (issue #133).
+  renderer — deliberately thin, just enough to display the
+  desktop + forward pointer + keyboard. A production-grade viewer
+  would vendor noVNC; this file ships the minimal in-tree variant).
+  Only accessible when `hyperhive.gui.enable = true` in the agent's
+  `agent.nix`; the harness shows a 🖥 screen link in the state row
+  when `gui_vnc_port` is present. Toolbar: `⤢ fit` CSS-downscales
+  the canvas to the window via `relayoutCanvas()` setting explicit
+  pixel dimensions on the canvas — *not* CSS `max-width/max-height`,
+  because a flex item's automatic minimum size (`min-width: auto`
+  resolves to the canvas's intrinsic framebuffer resolution) silently
+  clamps `max-*` back up, making fit mode a no-op that just centred +
+  clipped the oversized canvas. The fit-mode rules pin the canvas
+  with `flex: none; min-width: 0; min-height: 0` so the JS-set size
+  sticks. `⤡ match size` sends an RFB `SetDesktopSize` request so the
+  server (weston) changes its real output resolution to the window
+  dimensions; enabled once the server advertises the
+  `ExtendedDesktopSize` pseudo-encoding (`-308` rect in the header).
+  Fit-mode state persists in `localStorage` (`screen-fit`); default
+  is on. Pointer coordinates are rescaled in `sendPointer` so clicks
+  land on the right pixel regardless of CSS scale.
 - `GET /screen/ws` — raw RFB byte relay: proxies WebSocket
   frames to the weston VNC server at `127.0.0.1:<vnc_port>`.
   Transparent to any RFB variant. VNC port comes from