docs: add implementation patterns & gotchas guide

[jfreed-dev]

Summary

Recovers durable libcosmic/iced development lessons from the project's pre-public history (the old docs/development/LEARNINGS.md) and rewrites them against the current codebase. The original note referenced files that no longer exist (wallpaper_config.rs, the monolithic app.rs, ThumbnailCache, flex_row/button::image), so it's reconstructed rather than restored.

New: docs/development/PATTERNS.md

• Performance — never work in view(): it runs every frame; cache results and failures; GPU image cost is decode+upload, not display size. Framed as a general principle (the wallpaper grid that taught it is gone).
• Live widget idioms, each grounded in current src/:
  • widget::dropdown — the 'static closure gotcha (src/app/visuals.rs)
  • file_chooser open/save dialogs with Cancelled handling
  • cosmic::task::future for async work off the UI thread
• Theming via the typed cosmic_theme API (dark_config()/light_config()), not hand-edited RON files.
• Config paths centralized in src/paths.rs.
• .ftl gotcha: RustEmbed embeds translations at compile time (src/localize.rs); cargo clean forces a re-embed.

Changed: docs/architecture/OVERVIEW.md

Cross-reference from "Performance Guidelines" to the new guide.

Notes

• Deliberately dropped the old PROJECT-NAMING.md (settled decision) and stale/removed widget examples to avoid re-introducing dead references.
• Docs-only; no code changes. lint-docs only checks cargo doc, so no markdown lint impact.
• Full original history preserved on the local pre-public-history branch.

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