[GEN-06] Input elements (welcome / channel rename / command palette) lack accessible labels

[intendednull]

Commit: 2f26d91 · Finding: GEN-06

Problem

Several <input> elements rely on placeholder only, violating WCAG 1.3.1 / 3.3.2:

• crates/web/src/components/welcome.rs:43-51 — display-name input.
• crates/web/src/components/channel_sidebar.rs:322 — channel rename input.
• crates/web/src/components/command_palette.rs:605 — palette search.

Screen readers announce these as "edit blank"; placeholder text is not a substitute for a label.

Fix

Add aria-label="..." (or associate with an adjacent heading via aria-labelledby="<heading-id>") on each of the three inputs.

Obvious fix — will be auto-PR'd.