[opariffazman]

Stacked on #10. The base is `feat/presets`, so the diff shows only this change. Retarget to `main` after #10 merges.

What changes

• Each monitor shows its own layout and preset. The page sends `?screen=KEY` on every request, and the server keeps one stage per monitor in `screens.json` (up to 16).
• A new monitor starts as a copy of the default stage. A lone monitor uses the default stage, so single-screen setups behave as before.
• A command with no `screen` reaches every stage. A named screen reaches that stage only, and an unknown name answers 404.
• New commands: `next-preset`, `prev-preset`, `screens`. They step from the preset the stage loaded last, wrap, and skip a preset with no model left.
• CLI: `--screen KEY` and `--screen current`. `current` reads the active monitor from KWin, so a global shortcut cycles the monitor in use.
• The Presets pane has Previous and Next buttons and highlights the preset loaded last. Local mode gets the same commands.

Monitor keys

The page builds the key from the screen size and position when `screen.isExtended` is true, for example `3440x1440+0+0`. `?screen=NAME` overrides it. A moved monitor gets a new key and starts again from the default layout.

Tests

• Python: 355 tests pass (28 new in `tests/server/test_screens.py`).
• Page: 79 tests pass (4 new in `screens.spec.mjs`, 1 new in `local.spec.mjs`).
• shellcheck passes.
• Checked on the real desktop with two monitors: each steps its own preset, and `--screen current` moves only the monitor the pointer is on.

Notes

• KDE's shortcut service keeps the command line it first read for a shortcut file. Changing `Exec` later has no effect until the next login. Register a shortcut under a new file name instead. This is why docs name `--screen current` in the command to bind.
• `save-preset` without `--screen` saves the default layout. Name the monitor on a multi-monitor desktop.