docs: add native frontend architecture implementation plan

[outlaw-dame]

Summary

Adds an internal implementation document for migrating Memory's frontend toward a Framework7-first, OS-adaptive, native-feeling UI architecture.

What changed

• Adds .gitignore coverage for docs/internal/ so future local internal planning notes are not accidentally added.
• Adds docs/internal/frontend-native-architecture-implementation.md as the reviewed implementation reference.
• Documents the proposed architecture around Vue 3, Framework7, Capacitor, Tailwind as token/layout utility, and Iconoir behind a semantic icon adapter.
• Includes OS/capability detection guidance for leaning on native icons where legally/practically available, native emoji, native keyboard behavior, haptics, safe areas, share sheets, PWA display modes, gestures, and motion.
• Includes phased migration plan, testing strategy, accessibility/security requirements, dependency policy, and acceptance criteria.

Notes

This PR is documentation-only. The branch originally had one interrupted placeholder file; it was removed before opening this PR, and the final diff only contains .gitignore plus the intended implementation document.

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive implementation plan for migrating the frontend to a Framework7-first, OS-adaptive semantic architecture, and updates the .gitignore file to exclude internal documentation drafts. Feedback on the plan identifies an invalid HTML attribute value for enterkeyhint ("newline") in the composer examples, which should be corrected to "enter" to align with the HTML specification.

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[gemini-code-assist]

According to the HTML specification, newline is not a valid value for the enterkeyhint attribute. The allowed values are enter, done, go, next, previous, search, and send. To indicate inserting a new line, use enter instead.

Suggested change

	<textarea enterkeyhint="newline" autocapitalize="sentences" autocorrect="on" spellcheck="true"></textarea>
	<textarea enterkeyhint="enter" autocapitalize="sentences" autocorrect="on" spellcheck="true"></textarea>