Add markdown export with ISO 8601 timestamped filenames

[Copilot]

Enables users to export voice notes to markdown files in device storage with timestamped filenames.

Changes

UI

• Added export button between notes view and voice input button

Export Logic

• Exports to Documents/VoiceNotes/voice-notes-{ISO-8601}.md
• Filename: yyyy-MM-dd'T'HH-mm-ss (filesystem-safe)
• Content timestamp: yyyy-MM-dd HH:mm:ss (human-readable)
• Validates notes exist before export
• Toast feedback for success/error states

Permissions

• WRITE_EXTERNAL_STORAGE with maxSdkVersion="28" for legacy Android
• API 29+ uses scoped storage (no explicit permission)

Example Output

# Voice Notes Export

**Exported on:** 2026-01-01 16:30:45

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[10:30:15] Pick up groceries

[10:35:42] Call dentist tomorrow morning

Implementation Notes

• ISO 8601 pattern extracted to companion constant
• Separate timestamp formatting for filename vs content readability
• Directory created automatically if missing

Original prompt

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This section details on the original issue you should resolve

<issue_title>Export notes to a markdown file in device storage</issue_title>
<issue_description>As a user, I'd like to be able to export the notes to a markdown file in the device memory.

• A default location should be provided by default, with the option to change it to a custom one if desired.
• The file could be timestamped as well (iso 8601 format)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Export notes to a markdown file in device storage #6

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