Improve Skychat on UI, Groups and Channel feature

[mrpalide]

Did you run make format && make check? Yes

Fixes #_

Changes:

• Replaced the sidebar PK input and the two group icons with a single button opening Add by address / New group / New channel — in the header on desktop, a floating button at the bottom on a phone.
• Added an Add by address dialog: one field takes a public key, a skychat:// address, an invite link, or a scanned QR code, resolves it, then offers exactly one action — Start Chat, Join Group, Send Request, or Join Channel.
• Added the skychat:// / skychat:/// address grammar as a new pkg/skychat/address package (100% covered).
• Added a describe-only probe on a new well-known dmsg port so a short address can be resolved and joined; it decides nothing, and a real join request sent there is refused.
• Added GroupResolve RPC, a /group/resolve route, and join-by-address through the visor, the app's HTTP proxy, and the CLI (group address, group resolve, --kind).
• Added channels as a real group kind — open to join, admins-only to post, permanently rather than via the reversible read-only flag; carried through invites and join responses so a joined channel can't degrade into an ordinary group.
• Added a QR view for the open conversation, with the address underneath as selectable text plus a copy button; scanning works from the camera or a chosen image, and says so plainly where the browser lacks the API.
• Made the layout one-pane below 760px: the list is the screen, opening a chat replaces it and grows a back button, and the device back button returns to the list.
• Documented all of it in the app README.
• Locked a channel's kind at the store boundary — it can never become a group, including via a write that merely omits the kind.
• Rewrote the subscription topology for channels so cost is flat in audience size instead of growing with every subscriber.
• Made channels serve history and files from their admins only, and refused the peer-backfill setting there rather than accepting it and doing nothing.
• Added a backward history cursor to both storage backends, threaded through the RPC and HTTP layers.
• Made the UI load a group or channel's backlog a chunk at a time — newest page on open, older pages as you scroll back.
• Made channel attachments reference-only: an unrequested file is refused even from an admin, so bytes arrive only when someone opens the card.
• Added an opt-in discovery catalog — a new wire frame on the existing probe port, plus RPC, HTTP route, CLI commands, a create-time checkbox and an admin toggle.
• Surfaced discovery where it's useful: entering a public key in Add by address also lists the channels that visor publishes.
• Added tests for the channel invariants and the page cursor, and documented all of it in the app README.

How to test this PR:

make e2e-build
make e2e-run
make e2e-skychat

then open http://localhost:8001 and http://localhost:8002 on different browser and check new changes.