feat: configurable ring settings screen

Adds a Settings screen (SettingsActivity) where the user configures:
- the SMS trigger phrase (replaces hardcoded RING_NOW/PLAY_SONG)
- the sound: system ringtone (with picker) or a picked audio file
- the play duration (5-300s, replaces the fixed 30s)

Values persist via RingConfig (SharedPreferences). SmsReceiver matches the
configured phrase; RingService plays the stored Uri for the configured length.
Removes the old MediaStore by-name song search and its hardcoded duration.

README.md

@@ -9,18 +9,26 @@ An Android app that listens for SMS commands to trigger a loud ring — overridi
 
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-## SMS Trigger Commands
+## SMS Trigger
 
-| Command            | Behaviour                                                                        |
-|--------------------|----------------------------------------------------------------------------------|
-| `RING_NOW`         | Plays the default system ringtone at max volume.                                  |
-| `PLAY_SONG <name>` | Searches local `MediaStore` for a matching song. Plays it, or falls back to the   default ringtone if not found.                                     |
+The app listens for a **configurable trigger phrase**. When an incoming SMS body
+matches it exactly (case-insensitive, trimmed), the app rings.
 
-Playback always runs for exactly **30 seconds**, then the previous volume is
-restored.
+> Both the phrase, the sound it plays, and the duration are configurable from the
+> in-app **Ring Settings** screen (launched from the **"Ring Settings"** button).
+> Send the configured phrase to your phone from any other phone to trigger it.
 
-> Send the SMS to your phone from any other phone. The command is matched
-> case-insensitively.
+### Trigger Phrase (default: `RING_NOW`)
+Configured in **Settings**. Any SMS whose body equals the phrase triggers the ring.
+
+### Sound
+Choose between:
+- **System ringtone** (or pick a specific one), or
+- **Custom song** — pick any audio file on the device.
+
+### Duration
+Playback runs for the configured number of seconds (`5–300`, default **30**), then
+the previous volume is restored.
 
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