HSL: click a colour chip to open its panel, the way PICK does

The colour band chips only aimed the mixer before; now they open the same
panel the eyedropper opens, seeded with the colour the chip is named after
(hexToRgb turns that readout back into the RGB bytes the picker stores).
A pick already on the photo keeps its spot — only a mixer with nowhere to
hang takes the middle of the frame.

The HSL column itself is untouched: 8 chips, the rule, IMAGE,
HUE/SAT/LUM, the readout and RESET all stay where they were.

docker/frontend/src/Landing.tsx

@@ -613,14 +613,14 @@ export function Landing() {
                       className={`chip${mode === 'light' ? ' on' : ''}`}
                       onClick={() => setMode('light')}
                     >
-                      Light
+                      <span className="chip-label">Light</span>
                     </button>
                     <button
                       type="button"
                       className={`chip${mode === 'dark' ? ' on' : ''}`}
                       onClick={() => setMode('dark')}
                     >
-                      Dark
+                      <span className="chip-label">Dark</span>
                     </button>
                     <button
                       type="button"
@@ -628,7 +628,7 @@ export function Landing() {
                       data-key="theme-auto"
                       onClick={() => setMode('auto')}
                     >
-                      Auto
+                      <span className="chip-label">Auto</span>
                     </button>
                   </div>
                   {/* The same colour groups the workspace offers; the page's own
@@ -642,7 +642,7 @@ export function Landing() {
                         className={`chip${accent === entry.id ? ' on' : ''}`}
                         onClick={() => setAccent(entry.id)}
                       >
-                        {entry.label}
+                        <span className="chip-label">{entry.label}</span>
                       </button>
                     ))}
                   </div>