feat: add "MiRA at a glance" figure and rework the splash actions

[shainova]

Why MiRA page:

• New "At a glance" section with the conceptual figure (load -> seven modes -> save), carrying a full descriptive alt so the content stays crawlable.
• Move the app screenshot down into "See it in action", under the tutorial cards, so the two large visuals no longer stack.

Splash screen:

• Drop the lab attribution line; replace the single guidelines link with three actions: "MiRA at a glance", "See the manual", "Load example data".
• "MiRA at a glance" opens the figure in a modal (X, backdrop click, and Esc all close it); "Load example data" opens the existing example-data dialog.
• Extract _drawSplashActions() so each action records its own hit box.

Fixes found while wiring this up:

• InteractionHandler reset the canvas cursor on every mousemove and registered after the splash handler, so the action links never showed a pointer. It now returns early when no network is loaded, matching the renderer's own splash condition (!this.model), leaving hover with a loaded network untouched.
• .dialog-box hard-codes width:360px, which a max-width cannot override, so the figure modal rendered tiny; .glance-dialog-box now sets an explicit width.