Rename "Folders" to "Memory" in UI

[mlund01]

Summary

Mirrors squadron#102, which renamed the HCL block names (shared_folder → shared_memory, folder → memory, run_folder → run_memory, folders = → memories =) while keeping the old spellings as backward-compatible aliases.

This PR flips the user-facing terminology in the command center to match. Scope is intentionally narrow — only displayed strings change.

Changes

• Sidebar nav (AppSidebar.tsx:125): Folders → Memory
• File browser page (FileBrowserPage.tsx):
  • Heading Folders → Memory
  • Empty-state copy now references shared_memory / memory blocks (the new HCL spellings)
• Tools page banner (PluginsPage.tsx:124): "shared folders are configured" → "shared memory is configured"

What didn't change (and why)

Matching squadron#102's "Go types stay" approach:

• Internal TS types (SharedFolderInfo, sharedFolders, folders in the wire payload) — the squadron-wire protocol (v0.0.41) still uses these. They become candidates for a follow-up once squadron-wire renames them.
• Routes (/files, ?browser=) — squadron kept doc URLs stable; same reasoning here.
• "Download folder" button in the file browser — refers to zipping a filesystem subdirectory inside a memory, not the memory concept itself.
• "Add file to folder" in ConfigPage — unrelated; refers to a directory in the on-disk config-file tree.

Test plan

• npm run build passes
• Manual: sidebar shows "Memory" when an instance has shared memory configured
• Manual: file browser page heading reads "Memory"; empty state mentions shared_memory / memory
• Manual: tools page banner reads "shared memory is configured"