Feature: Share messages and links (copy, Web Share, /share route)

[mjkatgithub]

Planning

• Milestone: Phase 3
• Priority: P2
• Size: M
• Estimate: 3 SP

Goal

Share content into Matrix and out of Decentra — distinct from in-app Forward (#105).

Background — Forward vs Share (Element)

• Forward (Feature: Forward messages to another room #105): existing Matrix event → another room.
• Share (this issue): bring external or link-based content into Matrix, or share out via OS/browser:
  • Inbound: Element /#/share?format=text&msg=… composes a new message via room picker (PR #29874).
  • Outbound: copy message permalink / matrix.to, Web Share API where available.
  • Room invite / "share room" links (QR, link copy) may overlap space invite UX — scope carefully.

Scope (phased — ship incrementally)

P1 — Copy link

• Copy link to message (matrix.to or client permalink if supported).
• Copy matrix.to user/room links where already used in onboarding.

P2 — Web Share API (optional)

• Share message text/link via navigator.share when supported.

P3 — Inbound share route (optional)

• /share route: accept msg (+ optional format), open room picker, send new message.

Out of scope

• Forward existing event to another room → Feature: Forward messages to another room #105.

Branch

feature/message-share

Acceptance Criteria

• At least P1 delivered: user can copy a shareable message link from UI.
• Document which Element share flows are implemented vs deferred.
• i18n EN/DE for share actions.

Test Checklist

• Copy link produces valid URL; manual open works where homeserver allows.
• Optional: /share route with logged-in user sends to chosen room.

Notes

• Reuse room-picker pattern from Feature: Forward messages to another room #105 where possible.

Epic

Sub-issue of #3 (Phase 3).