feat(swa): add sidebar navigation to group sections

[Ryback2501]

Summary

• Add a left icon sidebar that organizes the logged-in UI into four sections, showing only the selected section at a time.

Changes

SWA (swa/)

• index.html — add a left sidebar rail with four buttons, each an inline single-<path> SVG icon (User, Relays, Chats, Feeds), wrapped with <main> in an app-shell. Group the panels into four .section containers: User (Identity + Profile + Following), Relays, Chats (Direct Messages), Feeds (Post + Feed). The Following panel was moved up to sit with the User group.
• style.css — lay out the sidebar rail beside the content via .app-shell; add .sidebar / .sidebar-btn (hover + active states) and .section styles; main now flexes within the shell.
• navigation.js — new pure module: section keys, default section, and resolveSection to pick a valid active section (with fallback).
• navigation.test.js — unit tests for resolveSection and the default-section invariant.
• app.js — wire icon clicks to show one section and mark its button active; show the sidebar only when logged in and reset to the default section on logout.

Test plan

• Log in and confirm the left icon rail appears with four icons.
• Click each icon and confirm only that section's panels show (User shows Identity/Profile/Following; Relays; Chats; Feeds shows Post/Feed) and the icon highlights.
• Log out and confirm the sidebar disappears; log back in and confirm it returns on the User section.
• Confirm the icons render as SVG shapes and are keyboard-focusable with accessible labels.

[Ryback2501]

Reopening from correctly-named branch feature/swa-sidebar-nav (branch-name gate requires the feature/ prefix).