Port "Mein Zimmer" - the Home Assistant room panel

The second of the web front-end's three pages. Lights get a toggle, five brightness
steps and six colours; shutters get four presets, up/stop/down and a window that
reflects the position Home Assistant reports. Its own violet hue rather than the
player's teal, so the two pages read as siblings rather than one bleeding into the
other - the same reason room.css has its own tokens.

The backend keeps Home Assistant's URL and token and relays the calls, so this client
learns only which entities exist, exactly as the browser does. A 404 from GET /api/ha
means Home Assistant is not configured, which is a normal answer: the tab simply does
not appear.

Polled, not subscribed, and only while the page is on screen. HA's own auth-and-
subscribe websocket is more machinery than a room panel needs, and a lamp's state is
not worth a request every 2.5 seconds when nobody is looking at it - this device has a
music player to stay out of the way of. A tap patches the entity locally and records
when; a poll that was already in flight when the patch landed is dropped for that
entity rather than putting the lamp back to "off" until the next one catches up. That
reconciliation is the one non-obvious thing in useHomeAssistant.ts and it is carried
over for the same reason.

Nothing on a shutter card animates locally. A real cover reports its own movement and
position, and a client-side guess at where it will end up is precisely what would
fight with that - the design mockup animated it because it had no backend to ask.

Two additions beyond a straight port, both because a kiosk has no pointer:

CTRL+TAB cycles the pages. The web's tab rail is pointer-only, which sits badly with a
front-end whose README claims every screen is reachable from the keyboard. Plain TAB
was already the group cycle, and CTRL+TAB is the idiom for the outer one everywhere
else.

--page opens directly on a page, which a kiosk may well want and which is the only way
in with neither keyboard nor pointer.

Verified against the real Home Assistant: the shutter reported open and the lamp off,
and both drew that way. Nothing was switched - those entities are hardware.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

slint-frontend/ui/models.slint

@@ -89,3 +89,63 @@ export struct NowPlaying {
     // "Noch 38:20 im Album" - empty for a podcast, where it means nothing.
     remaining-label: string,
 }
+
+// ------------------------------------------------------------- Mein Zimmer --
+
+// One device card on the room page. Lights and shutters share a struct rather than
+// having one each, because the grid renders them in the order Home Assistant's config
+// lists them and two models would lose that order. `kind` is what the view branches on.
+export struct RoomCard {
+    entity-id: string,
+    name: string,
+    // "light" | "shutter"
+    kind: string,
+    on: bool,
+
+    // --- light ---
+    is-color: bool,
+    // 0..5, matching the five brightness buttons.
+    level: int,
+    tint: color,
+    // Which of the six swatches the light is currently set to, or -1.
+    swatch: int,
+
+    // --- shutter ---
+    // Percent *closed*: 0 is fully open. Home Assistant reports the opposite; the
+    // inversion happens once, in Rust.
+    closed-percent: float,
+    // "Fährt runter …", or the preset name the position matches.
+    status: string,
+    // "" | "up" | "down"
+    moving: string,
+    supports-position: bool,
+    // Which of the four presets the position matches, or -1.
+    preset: int,
+}
+
+// The room grid, pre-chunked into rows for the same reason the album grid is: the
+// view lays rows out, and how many fit is a function of the window width, which Rust
+// is the one that knows.
+export struct RoomRow {
+    cards: [RoomCard],
+}
+
+export struct SceneTile {
+    entity-id: string,
+    name: string,
+    active: bool,
+}
+
+// One of the six colours a colour-capable light can be set to.
+export struct Swatch {
+    name: string,
+    tint: color,
+}
+
+// One entry in the vertical tab rail. Built in Rust like every other string on
+// screen, so the page order, the icons and the German labels live in one place.
+export struct Tab {
+    id: string,
+    icon: string,
+    label: string,
+}