feat(ui): compact layout for small terminal windows (#37)

In a short window the fixed root layout runs out of rows and the CPU graph and
per-core bars are what collapse first: the header, gauges and process table hold
fixed heights, so at ~18 rows the top row is left with no drawable interior and
both panes disappear entirely.

Add a second layout, entered automatically once the Disks pane can no longer show
even one complete disk card:

- Disks is dropped — it is the pane that degrades worst when partly drawn.
- Memory and Swap move side by side into the row Disks vacated.
- GPU collapses to a single full-width line (utilisation and VRAM, no device
  name), and is omitted entirely on a host that reports no GPU.
- The reclaimed rows go to the CPU panes, with the surplus above their floor
  shared with the bottom half so the process table still grows with the window.

`--compact` pins the layout at any size.

The root layout was duplicated in three places (the draw path and both input
hit-testing paths), which would have drifted the moment a second layout existed.
Move it into ui::layout as the single source of truth and have all three callers
go through it.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -31,6 +31,8 @@ socktop is a remote system monitor with a rich TUI, inspired by top/btop, talkin
   - Only top-level processes listed (threads hidden) — matches btop/top
 - Optional GPU metrics (can be disabled)
 - Optional auth token for the agent
+- Compact layout for small windows: automatically drops the panes that no longer fit so
+  the CPU graph and per-core bars stay visible (see [Compact mode](#compact-mode))
 
 ---
 
@@ -213,6 +215,8 @@ socktop --verify-hostname --tls-ca /path/to/cert.pem wss://HOST:8443/ws
 # shorthand:
 socktop -t /path/to/cert.pem wss://HOST:8443/ws
 # Note: providing --tls-ca/-t automatically upgrades ws:// to wss:// if you forget
+# force the small-window layout at any terminal size (normally automatic):
+socktop --compact ws://HOST:3000/ws
 ```
 
 Intervals (client-driven):
@@ -224,6 +228,29 @@ The agent stays idle unless queried. When queried, it collects just what’s nee
 
 ---
 
+## Compact mode
+
+In a short terminal the fixed layout runs out of rows and the CPU graph and per-core bars
+are the first things to collapse — exactly the panes you are most likely watching. Once
+the window is too short for the Disks pane to show even one disk, socktop switches to a
+compact layout:
+
+- **Disks is dropped.** It is the pane that degrades worst when partially drawn.
+- **Memory and Swap move side by side** into the row Disks vacated.
+- **GPU shrinks to a single line** — utilisation and VRAM only, no device name. On a host
+  with no GPU the pane disappears entirely.
+- **Everything reclaimed goes to the CPU graph and per-core bars**, which stay usable well
+  below the size where they used to vanish.
+
+The switch is automatic and needs no configuration. Pass `--compact` to pin the compact
+layout at any window size:
+
+```bash
+socktop --compact ws://HOST:3000/ws
+```
+
+---
+
 ## Connection Profiles (Named)
 
 You can save frequently used connection settings (URL + optional TLS CA path) under a short name and reuse them later.