Add select-all command for web graph elements

[joanjcaceres]

Context

While testing PR #87, Joan noted that the web app would benefit from a clear way to select all graph elements.

The app already supports box selection and multi-selection workflows, but selecting a whole circuit currently requires manual drag selection, which is awkward for large projects.

Proposal

Add a select-all command for the web app.

Acceptance Criteria

• Add a visible toolbar or menu control to select all graph elements.
• Support a standard keyboard shortcut, preferably Cmd+A / Ctrl+A, when focus is not inside an editable field.
• Select nodes and edges in a way that works naturally with existing copy, delete, concatenate, and merge workflows.
• Do not intercept native select-all behavior inside text inputs, textareas, or editable fields.
• Add E2E coverage for the button and shortcut.

Related

• Follow-up from testing PR Improve analysis sweep responsiveness and plot layout #87.