Add Fastlane metadata for F-Droid (title, descriptions, changelog, screenshots)

F-Droid's RFP scanner flagged the repo for missing Fastlane metadata
(https://gitlab.com/fdroid/rfp/-/work_items/4395#note_3827250669):
without it, F-Droid can't show app screenshots and description edits
require going through the F-Droid team directly.

Screenshots are symlinked from docs/assets/screenshots/android/ rather
than duplicated, keeping one source of truth. Also drops the stock
fastlane .gitignore rule that was silently excluding these PNGs --
unlike Play Store screenshots, F-Droid reads them straight from the
repo with no separate regeneration step, so they're meant to be
committed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy

fastlane/metadata/android/en-US/changelogs/13.txt

@@ -0,0 +1,16 @@
+Rewritten from the ground up as a Kotlin Multiplatform app - same features, now with a shared codebase across platforms.
+
+New:
+- Country flag emoji next to each currency code, where available
+- Support for precious metals (gold, silver, platinum, palladium) alongside fiat currencies
+- Search in the currency picker, with results also matching issuing country names
+- Currency picker now splits selections into Priced/Unpriced
+- Reset button to clear all selected currencies (with confirmation)
+- "1 currency to Sats" field, editable directly for Manual price sources
+- Move currencies to the top or bottom of the list
+
+Improved:
+- Manual price source: simplified UI, seeded from another source, editable rate
+- Selected currencies, exchange rates, and price source now persist between launches
+- Locale-aware number formatting (digit grouping) and human-readable "updated" timestamps
+- More accurate currency list: withdrawn/superseded currency codes filtered out