Env var list should show one row per variable, not one row per unique value

Currently, the env var list collapses or splits rows based on whether
the value is identical across environments:

  • Same value in dev + prod → one row
  • Different values → two rows

This breaks the core mental model. The number of rows in the list no
longer equals the number of variables. Scanning becomes unreliable
because you can’t tell at a glance how many variables you have or
whether a variable exists in a given environment.

Proposed behavior:

  • Always show one row per variable name
  • Clicking a row opens a modal
  • The modal shows each environment with its current (masked) value
    and an input to update it
  • All edits to one variable happen in one place

This keeps the list scannable and predictable. The “collapse identical
values” optimization can still happen inside the modal if space is a
concern, but it shouldn’t affect the list itself.