Initial commit: Shelly Manager with Textual CLI, Streamlit UI, and comprehensive .gitignore

Shelly device management app with mDNS/subnet discovery, inventory,
configuration, and mass operations for Gen1/Gen2+ devices.

Includes .gitignore excluding runtime data (device DB, user config),
AI conversation history, build artifacts, and common Python/OS patterns.

tests/test_config_snapshot_diff.py

@@ -0,0 +1,84 @@
+"""Config snapshot unified diff (GetConfig history)."""
+
+from __future__ import annotations
+
+from datetime import UTC, datetime, timedelta
+
+from shelly_manager.core.config_snapshot_diff import (
+    pick_older_newer,
+    settings_unified_diff_text,
+    sort_snapshots_chronologically,
+)
+from shelly_manager.core.models import ConfigSnapshot
+
+
+def _snap(
+    *,
+    sid: str,
+    created: datetime,
+    label: str,
+    settings: dict,
+) -> ConfigSnapshot:
+    return ConfigSnapshot(
+        id=sid,
+        device_id="DEV",
+        created_at=created,
+        label=label,
+        settings=settings,
+    )
+
+
+def test_sort_snapshots_chronologically_oldest_first() -> None:
+    t0 = datetime(2026, 3, 20, 0, 37, tzinfo=UTC)
+    t1 = datetime(2026, 3, 20, 0, 42, tzinfo=UTC)
+    a = _snap(sid="a", created=t1, label="newer", settings={})
+    b = _snap(sid="b", created=t0, label="older", settings={})
+    out = sort_snapshots_chronologically([a, b])
+    assert [x.label for x in out] == ["older", "newer"]
+
+
+def test_pick_older_newer_swaps_when_user_picks_newer_as_a() -> None:
+    """Oldest-first list: index 0 older, index 1 newer — if user picks A=1 B=0, still get older→newer."""
+    t0 = datetime(2026, 3, 20, 0, 37, tzinfo=UTC)
+    t1 = datetime(2026, 3, 20, 0, 42, tzinfo=UTC)
+    snaps = [
+        _snap(sid="b", created=t0, label="older", settings={"x": 1}),
+        _snap(sid="a", created=t1, label="newer", settings={"x": 2}),
+    ]
+    older, newer = pick_older_newer(snaps, 1, 0)
+    assert older.label == "older"
+    assert newer.label == "newer"
+
+
+def test_settings_unified_diff_text_has_line_separated_structure() -> None:
+    """Regression: diff must contain newlines so each +/- line is separate (not one wrapped blob)."""
+    old = {"ble": {"enable": True}}
+    new = {"ble": {"enable": False}}
+    text = settings_unified_diff_text(old, new, fromfile="older/settings.json", tofile="newer/settings.json")
+    assert text.count("\n") >= 4
+    lines = text.splitlines()
+    assert lines[0].startswith("--- ")
+    assert lines[1].startswith("+++ ")
+    assert any(line.startswith("@@") for line in lines)
+    # Every content/hunk line must be its own line (no embedded @@ mid-line)
+    for line in lines:
+        assert "\n" not in line
+    minus_lines = [ln for ln in lines if ln.startswith("-") and not ln.startswith("---")]
+    plus_lines = [ln for ln in lines if ln.startswith("+") and not ln.startswith("+++")]
+    assert minus_lines
+    assert plus_lines
+
+
+def test_settings_unified_diff_identical_empty() -> None:
+    cfg = {"a": 1}
+    assert settings_unified_diff_text(cfg, cfg) == ""
+
+
+def test_pick_older_newer_same_timestamp_tie_break() -> None:
+    t = datetime(2026, 3, 20, 12, 0, tzinfo=UTC)
+    s1 = _snap(sid="aaa", created=t, label="first", settings={})
+    s2 = _snap(sid="zzz", created=t, label="second", settings={})
+    snaps = sort_snapshots_chronologically([s2, s1])
+    older, newer = pick_older_newer(snaps, 0, 1)
+    assert older.id == "aaa"
+    assert newer.id == "zzz"