feat(swa): move feed compose into a modal opened by a fixed button

[Ryback2501]

Summary

• Turn the Feeds compose area into a modal opened by a floating "Post" button fixed to the bottom-right of the feed, so it stays reachable regardless of feed scroll and no longer occupies a permanent panel above the feed.

Changes

Feed client (swa/)

• index.html — move the compose markup (subject, textarea, char counter, Post button, PoW selector, result) out of the always-present #post-panel into a new #post-modal reusing the existing modal pattern; add a fixed #open-post-btn trigger inside the Feeds section.
• app.js — wire the trigger to open the modal (focus the textarea); close on the close button, backdrop click, and Escape; close the modal after a successful post (errors keep it open).
• style.css — .post-fab fixed bottom-right pill button and a .post-modal-card width.

Test plan

• Log in, open Feeds — a floating Post button sits at the bottom-right; scrolling the feed keeps it visible.
• Click it — the modal opens with subject, textarea, char counter, PoW selector, and Post button, identical to the previous panel.
• Submit a note — it publishes, appears in the feed, and the modal closes; reopening shows cleared fields.
• Submit with no relay/keys — the error shows and the modal stays open.
• Switch to User/Relays/Chats — the floating button is hidden; return to Feeds — it reappears. Escape and backdrop click close the modal.