Add a config file, and size the grid to the display

Colours, border and thumbnail size come from ~/.config/wl-pick/config
now, since ten more flags would have made a keybinding unreadable — the
split the rofi setup this replaces already used: look in a file,
behaviour on the command line. Flat `key = value` lines, so no TOML
dependency for something with nothing to nest, and a flag still beats the
file.

Sizes take sway's units. `600px` is absolute; `70ppt` is a percentage of
the display the grid appears on, resolved on every run rather than baked
in, so one config suits monitors of different sizes. That needed the
overlay to know which display it is on, so it now asks sway for the
focused one and maps there explicitly, at that display's scale, instead
of letting the compositor choose and taking the largest scale in use —
which was wrong on any mixed-DPI setup.

tile-width and tile-height are maxima. Given only a width, the height
follows the display's aspect: a 16:9 cell, inherited from a rofi theme
written for a landscape screen, wasted about half of every cell on a
portrait monitor. And if the grid would outgrow the display, tiles now
shrink together, keeping their shape, so thirty windows produce small
tiles rather than a surface larger than the screen. The surface is capped
at the display as a backstop, because on a small screen the padding and
label rows can exceed it no matter how small the tiles get.

Two bugs the tests caught while writing this:

- Stripping comments at the first '#' ate colour values, so
  `selection = #d79921 # note` parsed as empty. A comment is now a '#'
  followed by whitespace or end of line; a colour is '#' then a hex
  digit, so the two cannot collide.
- Twelve tiles at the old fixed size fit a 1280x800 screen, so the first
  version of the shrink test proved nothing. It now uses numbers that
  genuinely overflow.

Also: failing to reach sway said only "No such file or directory", which
tells a first-time user nothing; it now names sway and what it wanted.

README.md

@@ -48,6 +48,7 @@ wl-pick [--format tsv|json|portal] [--live all|current|none] [--fps N]
 - `--hide-labels` draws an icon-only grid
 - `--font FAMILY` label font family (default: the system monospace font)
 - `--font-size PX` label size in logical px
+- `--config PATH` config file (default `~/.config/wl-pick/config`)
 - `--timeout SECS` exits after a deadline, in case the keyboard grab ever traps
   you
 - `--verbose` phase timings, the tile list, and capture stats
@@ -135,13 +136,53 @@ of shm for eight windows and a display here, against 83 MB with `--live none`)
 and a readback per refreshed frame. `--live current` refreshes only the selected tile,
 which is much cheaper and still reads as alive.
 
+## Config
+
+`~/.config/wl-pick/config`, or `--config PATH`. Flat `key = value` lines with
+`#` comments, everything optional, and a flag always beats the file. No TOML
+dependency, because there is nothing to nest.
+
+```ini
+background     = #282828      # the grid's backdrop
+foreground     = #ebdbb2      # label text
+selection      = #d79921      # the highlighted tile
+selection-text = #282828      # its label
+border         = #d79921
+border-width   = 2px
+
+tile-width     = 18ppt        # largest a thumbnail may be
+tile-height    = 20ppt        # defaults to the display's aspect
+max-columns    = 4
+
+font           = monospace
+font-size      = 13.3
+labels         = yes
+outputs        = yes
+live           = all
+fps            = 12
+format         = tsv
+```
+
+Sizes take sway's units: `600px` is absolute, `70ppt` a percentage — and the
+percentage resolves against **the display the grid actually appears on**, every
+time it runs. On a mixed setup one file gives 18% of a 1280-wide laptop panel and
+18% of a 3840-wide monitor, instead of a pixel count that suits one and looks
+wrong on the other. The overlay is mapped explicitly on that display, at that
+display's scale, so mixed-DPI renders crisply either way.
+
+`tile-width` and `tile-height` are maxima for the thumbnail cell. Give only the
+width and the height follows the display's aspect, which is roughly the shape of
+the windows on it — a 16:9 cell wastes about half its area on a portrait monitor.
+If the grid would outgrow the display, tiles shrink together and keep their
+shape, so thirty windows give small tiles rather than a surface larger than the
+screen.
+
 ## Look
 
-Colours, font metrics and grid geometry come from the rofi theme this replaces
-(gruvbox dark, a yellow selection filling the element padding, `ceil(sqrt(n))`
-columns capped at 4, 16:9 tiles, `title · app` centred underneath) and live in
-`src/theme.rs`, which is the one place to change them. They are not
-configurable at runtime beyond the font flags.
+The defaults come from the rofi theme this replaces: gruvbox dark, a yellow
+selection filling the element padding, `ceil(sqrt(n))` columns capped at 4,
+`title · app` centred underneath. Padding, gaps and margins are still fixed, in
+`src/theme.rs`.
 
 The label font defaults to the system monospace font — whatever `fc-match
 monospace` answers, which is what the rest of the desktop uses. (cosmic-text's