touch: raise SD max_files 3 -> 6; report chat-save failure stage + errno

Field evidence reframed the chat-save failures: a soft reboot wrote the
FULL 515 KB ring to the same card without trouble, while the periodic
worker autosave could not even create the file. That rules out a
size/bad-region card problem and points at contention: the SD VFS was
mounted with max_files=3 (set when the card only served the file
manager), but the card now serves telemetry log rewrites (2 handles),
battery log rewrites (2), the history flush, tile reads/writes, WAV
chimes and the About readout concurrently -- the flush's open("w") loses
that race with ENFILE while the card itself is fine, and at reboot
(everything quiesced) the same write sails through. All four mount
sites now use max_files=6 (~+1.7 KB while mounted).

To make the next such failure self-explaining on a device with no
readable serial, the About Chat-store line now includes the failure
stage and errno ('FAIL x3 @o e23'): o/h/b/r = open/header/body/rename;
errno 23/24 = VFS file table full, 28 = card full, 5 = card I/O error.
The header-write failure path also joins the failure accounting (it
returned early without counting before) and cleans up its temp file.

Builds verified: all five SD-relevant touch envs.

Signed-off-by: Dan Vybiral <dan.vybiral@greencode.cz>

src/main.cpp

@@ -520,7 +520,7 @@ void setup() {
       for (int a = 0; a < tries && !sd_mounted; ++a) {
         SD.end();
         delay(kBootMount[a].settle_ms);
-        if (SD.begin(PIN_SD_CS, *_spi, kBootMount[a].hz, "/sd", 3) && SD.cardType() != CARD_NONE)
+        if (SD.begin(PIN_SD_CS, *_spi, kBootMount[a].hz, "/sd", 6) && SD.cardType() != CARD_NONE)
           sd_mounted = true;
       }
     }