fix(gui): remember auto-gradient choice per project

[MoraesGil]

Description

Overview - #826

Closes #826.

The confirmation above four physical filaments is an intentional safeguard: pairwise gradients grow as n × (n - 1) / 2. This change keeps that safeguard and adds an explicit way to remember the answer for one project, instead of bypassing confirmation application-wide.

Changes Made

• Add Remember this choice for this project to the existing Yes/No dialog.
• Store both Yes and No in project configuration together with the matching physical-filament count.
• Reuse the answer only when reopening that saved project with the same count; ask again after the count changes.
• Mark the project dirty when a choice is remembered so the user can persist it in the .3mf.
• Keep project-only metadata out of print-preset dirty checks.
• Reset the legacy in-session answer cache when another project is loaded, preventing one file from affecting another file with the same count.

Preserved Safety Behavior

Auto-generate	Physical filaments	Matching project choice	Result
Off	Any	Any	Do not generate
On	0–4	Any	Generate without prompting
On	5+	None or different count	Ask
On	5+	Yes, same count	Generate
On	5+	No, same count	Do not generate

Screenshots/Recordings/Graphs

The recording uses an isolated macOS arm64 build and a seven-filament fixture. It shows the 7 → 21 safeguard, remembers Yes in the project, reopens the saved .3mf without prompting, then changes the physical count so the safeguard asks again for 8 → 28 gradients.

Verification

• Focused Catch2 suite: 8 test cases, 15 assertions — passed.
• Standalone C++17 policy contract with -Wall -Wextra -Werror — passed, including unchanged default behavior for counts 0–32.
• macOS arm64 Release target — built successfully from the final diff.
• git diff --check — passed.
• Project round-trip — the saved .3mf contains choice = 1 and physical_count = 7.
• CUA — read the checkbox back as selected before Yes, observed no Auto gradients window after reopening the saved project, then observed the exact 8 → 28 prompt after adding an eighth physical filament.
• Runtime isolation — used a temporary app bundle, isolated app data, and loopback-only networking; the installed Snapmaker Orca app was not modified.

The wider local [MixedFilament] run reaches an existing assertion in PrintRegion.cpp (wall_filament <= num_extruders) after 22 test cases / 142 assertions; that test and code path are unchanged by this PR. The repository's Ubuntu workflow also has a pre-existing Catch2 v2 include failure reproduced on the exact base commit.