Fix screen CS crosstalk window and add IRAM_ATTR to button ISRs

[Bullpuph123]

Split from #330 (part 3 of 3), rebased onto dev as requested.

Summary

• selectScreen: deselect-all-first: the previous single-pass loop wrote CS pins in order, which can transiently leave both the old and new panel selected, letting SPI data leak to the wrong orb during the transition (observed on hardware as stray pixels appearing on a different orb). Now all CS lines are raised first, then only the target is lowered. Also adds a bounds check on the screen index (logs a warning and leaves everything deselected instead of driving a wrong/out-of-range CS pin), and hoists the ConfigManager read out of the loop.
• IRAM_ATTR on button ISRs: Button::isrButtonChange() and the MainHelper ISR wrappers run from GPIO interrupts but weren't placed in IRAM. If the interrupt fires while flash cache is disabled (NVS/LittleFS write), the ESP32 crashes with a Cache disabled but cached memory region accessed exception. millis() and digitalRead() are IRAM-safe, so marking the handlers is sufficient.

Test plan

• Builds with pio run -e infoorbs-v0_3
• Press buttons during a config save (flash write) — no crash
• Verify widgets still render to the correct orbs with orbRotation 0-3

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves ESP32 firmware reliability by (1) preventing transient SPI chip-select overlap when switching between TFT panels and (2) ensuring GPIO button interrupt handlers execute from IRAM to avoid crashes during flash cache-disabled windows.

Changes:

• Updated ScreenManager::selectScreen() to deselect all CS lines first, validate the requested screen index, and only then assert the target CS (with rotation-aware mapping).
• Marked button ISR entrypoints (MainHelper ISR wrappers and Button::isrButtonChange()) with IRAM_ATTR and documented the rationale (flash write / cache-disabled crash avoidance).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
firmware/src/core/utils/MainHelper.cpp	Marks the attachInterrupt ISR wrappers as IRAM_ATTR and clarifies ISR/IRAM requirements in comments.
firmware/src/core/screenmanager/ScreenManager.cpp	Makes screen selection a two-step “deselect-all then select-one” operation and adds an invalid-index guard + warning log.
firmware/src/core/button/Button.cpp	Marks the button edge handler as IRAM_ATTR so the ISR path runs from IRAM during flash cache-disabled periods.

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