[FEATURE] Improve service UX: Replace mower_serial text input with dynamic dropdown selector

[WhyLev]

## Is your feature request related to a problem?

Currently, all services that target a specific mower require users to manually enter the mower's serial number as a text field. This creates several problems:

1. Typing errors – Users frequently mistype the 16-character alphanumeric serial number, causing service calls to fail with "No mower instance found" errors.
2. Poor user experience – Users must remember or look up their mower's serial number, which is not user-friendly.
3. No validation – The text field doesn't validate if the entered serial exists, leading to runtime errors.
4. Multiple mowers confusion – With multiple mowers, users have to know which serial belongs to which mower, increasing cognitive load.
5. Security risk – Users might accidentally target the wrong mower due to typos.

This is particularly frustrating for users who have multiple mowers or for new users who are still learning the integration.

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## Describe the solution you'd like

Replace the text input for mower_serial with a dynamic dropdown selector in all services:

UI Changes

Before (Text input):

┌──────────────────────────────────────────────────┐
│ Serialnumber: [______________________________]  │
└──────────────────────────────────────────────────┘

After (Dropdown):

┌──────────────────────────────────────────────────┐
│ Mower: [▼ 0123456789ABC ]                       │
│        ├─ 0123456789ABC (Front Garden)          │
│        ├─ 9876543210DEF (Back Garden)           │
│        └─ 1122334455GHI (Side Garden)           │
└──────────────────────────────────────────────────┘

Technical Implementation

1. Update services.yaml: Change all mower_serial selectors from text to select with dynamic options:

   mower_serial:
     name: Mower
     description: Select the mower you want to control.
     required: false
     selector:
       select:
         options: []
         custom_value: true
         mode: dropdown

2. Fallback logic: The existing find_instance_for_mower_service_call() already handles None correctly:

   • If only one mower is configured → automatically uses that mower
   • If multiple mowers are configured → dropdown shows all options

3. Service names: Change "Serialnumber" to "Mower" for better UX

4. Optional manual input: Keep custom_value: true so users can still manually type a serial if needed (e.g., for testing or scripts)

Services Affected

• command
• smartmowing
• delete_alert
• delete_alert_all
• read_alert
• read_alert_all
• download_map
• set_calendar_slot
• set_predictive_mowing_window
• border_cut

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## Describe alternatives you've considered

1. Keep text input but add validation – Could validate on the backend and return better error messages, but still requires manual typing.
2. Auto-detect single mower – Already implemented. If only one mower exists, use it automatically. But for multiple mowers, users still need manual input.
3. Add service targets – Home Assistant has service targets, but they are less flexible and don't solve the discovery issue.
4. Device-based service calls – Let users call services directly on the device entity (like Lawn Mower entity). This already works for some services but not all.

Why the dropdown is better:

• No typing required, eliminates typos
• Shows all available mowers clearly
• Users see the mower names (if configured)
• Consistent with Home Assistant's design patterns
• Works in UI, automations, and scripts

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## Additional context

User Benefits

• For single mower users: Field is optional, auto-selects the only mower
• For multiple mower users: Clear dropdown with all options visible
• For automation users: Can still use mower_serial as text in YAML (backward compatible)
• For new users: No need to find the serial number

Design Principles

• Discoverability: Users immediately see available mowers
• Error prevention: No typos possible
• Efficiency: One click instead of typing 16 characters
• Flexibility: Still supports manual entry if needed