Support energy-parameter entry for capacitance and inductance values

[joanjcaceres]

Context

Users often think in terms of charging and inductive energies (E_C, E_L) instead of capacitance and inductance values (C, L). cQEDraw currently asks for capacitance, linear inductance, and Josephson inductance directly, then generates C and L_inv matrix expressions.

Scope To Design

• Let users enter either physical component values (C, L, LJ) or equivalent energy values (E_C, E_L, E_J) where appropriate.
• Define the exact conversion conventions and units before implementation.
• Preserve the generated C_matrix, L_inv_matrix, and Josephson metadata contracts.
• Make it clear in the UI which representation is active for each component.
• Avoid silently mixing incompatible units or conventions.

Questions

• Should energy inputs be accepted only as numeric values, or also symbolic expressions?
• Which units should be used in the web UI by default: SI joules, Hz/GHz energies, or angular-frequency units?
• Should E_J be an alternative to LJ for Josephson branches in the saved project schema?
• How should copied Python snippets expose the original user-entered representation?
• Should this be implemented in cQEDraw only, or aligned with sccircuits APIs first?

Acceptance Criteria

• A documented conversion convention exists for C <-> E_C, L <-> E_L, and LJ <-> E_J.
• The project schema can round-trip whichever representation the user entered.
• Generated matrix expressions remain unambiguous and backwards-compatible.
• Web UI makes the active representation obvious per component.
• Tests cover numeric and symbolic inputs, save/load, copy/paste, generated matrices, and analysis.

Notes

This should be a separate feature PR from the current plot-readability work because it changes input semantics and saved project data, not just chart presentation.