feat(printer): add smart-plug power switch button (Issue #103); fix(update): stable update check missing behind prereleases (Issue #104)

Issue #103: the printer has no MQTT-level command to power off or enter
standby, so users on a separate-room setup have to physically walk over
or use a smart plug (e.g. Tasmota) manually. Added per-printer
power_on_url/power_off_url/power_status_url config (Settings > Power
Switch) and a power button with a live on/off indicator on each printer's
card in the Printers grid - plain HTTP GET calls, no Moonraker
device_power dependency.

Issue #104: the stable-release update check only ever requested the
single newest Gitea release (limit=1) regardless of type. Since
nightly/dev prereleases publish far more often than stable ones, that
newest release is almost always a prerelease, so the "not a prerelease"
filter found nothing and reported "no stable releases found" even
though a newer stable release existed further back in the list.
Fixed by requesting enough releases (limit=20) to look past a run of
prereleases.

Also fixes a related pre-existing bug surfaced while testing #103's
config: configparser's default string interpolation rejected any
config.ini value containing a literal '%' (ValueError: invalid
interpolation syntax) - this broke saving Tasmota-style power URLs
(cmnd=Power%20on) and would have broken any other value with a '%'
character. Fixed globally with interpolation=None on every
ConfigParser() instantiation in config_loader.py and
kobrax_moonraker_bridge.py.

tests/test_update_check.py

@@ -0,0 +1,57 @@
+"""Update-check regression for Issue #104.
+
+STABLE_RELEASE_API used limit=1, so it only ever saw the single newest
+release on Gitea regardless of type. Since nightly/dev prereleases publish
+far more often than stable releases, that newest release is almost always a
+prerelease - the stable_releases filter (not prerelease) then found nothing
+and /api/update/check returned "no stable releases found" even though older
+stable releases exist.
+"""
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+
+def _fake_releases_response(payload):
+    resp = MagicMock()
+    resp.status = 200
+    resp.json = AsyncMock(return_value=payload)
+    ctx = MagicMock()
+    ctx.__aenter__ = AsyncMock(return_value=resp)
+    ctx.__aexit__ = AsyncMock(return_value=False)
+    return ctx
+
+
+@pytest.mark.asyncio
+async def test_stable_update_check_finds_release_behind_newer_prereleases(client):
+    c, bridge = client
+    bridge._read_version = lambda: "0.9.27"
+
+    releases = (
+        [{"tag_name": f"nightly-0.9.30-nightly{i}", "prerelease": True} for i in range(1, 7)]
+        + [{"tag_name": "v0.9.29", "prerelease": False, "body": "changelog"}]
+    )
+
+    with patch("aiohttp.ClientSession.get", return_value=_fake_releases_response(releases)):
+        resp = await c.get("/api/update/check")
+        data = await resp.json()
+
+    assert resp.status == 200
+    assert data["latest"] == "0.9.29"
+    assert data["update_available"] is True
+
+
+@pytest.mark.asyncio
+async def test_stable_update_check_requests_enough_releases_to_skip_prereleases(client):
+    """The API URL itself must ask for more than the single newest release -
+    a limit=1 request can never find a stable release behind a run of
+    prereleases no matter how the response is parsed."""
+    c, bridge = client
+    bridge._read_version = lambda: "0.9.27"
+
+    import re
+    assert not re.search(r"limit=1(?!\d)", bridge.STABLE_RELEASE_API), (
+        "STABLE_RELEASE_API must request more than 1 release, otherwise a "
+        "recent nightly/dev prerelease being the newest release hides all "
+        "stable releases behind it (Issue #104)"
+    )